Note: Descriptions are shown in the official language in which they were submitted.
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NSC-M742
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DESCRIPTION
PEARLITIC STEEL RAIL EXCELLENT IN WEAR RESISTANCE AND
DUCTILITY AND METHOD FOR PRODUCING THE SAME
Technical Field
The present invention relates to: a pearlitic steel
rail that is aimed at improving wear resistance at the
head portion of a steel rail for a heavy-load railway,
enhancing resistance to breakage of the rail by improving
ductility through controlling the number of fine pearlite
block grains at the head portion of the rail, and
preventing the toughness of the web and base portions of
the rail from deteriorating by reducing the formation of
pro-eutectoid cementite structures at these portions; and
a method for efficiently producing a high-quality
pearlitic steel rail by optimizing the heating conditions
of a bloom (slab) for said rail, thus preventing cracking
and breakage during hot rolling, and suppressing
decarburization in the outer surface layer of the bloom
(slab).
Background Art
Overseas, in heavy-load railways, attempts have been
made to increase the speed and loading weight of a train
to improve the efficiency of railway transportation.
Such an improvement in the railway transportation
efficiency means that the environment for the use of
rails is becoming increasingly severe, and this requires
further improvements in the material quality of rails.
Specifically, wear at the gauge corner and the head side
portions of a rail laid on a curved track increases
drastically and the fact has come to be viewed as a
problem from the viewpoint of the service life of a rail.
In this background, the developments of rails aimed
mainly at enhancing wear resistance have been promoted as
described below.
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1) A method of producing a high-strength rail having
a tensile strength of 130 kgf/mm2 (1,274 MPa) or more,
characterized by subjecting the head portion of the rail
to accelerated cooling at a cooling rate of 1 to 4 C/sec.
from the austenite temperature range to a temperature in
the range from 850 C to 500 C after the end of rolling or
the application of reheating (Japanese Unexamined Patent
Publication No. S57-198216).
2) A rail excellent in wear resistance wherein a
hyper-eutectoid steel (containing over 0.85 to 1.20% C)
is used and the density of cementite in lamella in
pearlite structures is increased (Japanese Unexamined
Patent Publication No. H8-144016).
In the case 1) above, it is intended that high
strength is secured by using a eutectoid carbon-
containing steel (containing 0.7 to 0.8% C) and thus
forming fine pearlite structures. However, there is a
problem in that wear resistance is insufficient and rail
breakage is likely to occur when the rail is used for a
heavy load railway since ductility is low. In the case
2) above, it is intended that wear resistance is improved
by using a hyper-eutectoid carbon steel (containing over
0.85 to 1.20% C), thus forming fine pearlite structures,
and then increasing the density of cementite in lamellae
in pearlite structures. However, ductility is prone to
deteriorate and, therefore, resistance to breakage of a
rail is low as the carbon content thereof is higher than
that of a presently used eutectoid carbon-containing
steel. Further, there is another problem in that
segregation bands, where carbon and alloying elements are
concentrated, are likely to form at the center portion of
a casting at the stage of the cast of molten steel, pro-
eutectoid cementite forms in a great amount along the
segregation bands especially at the web portion
of a rail
after rolling, and the pro-eutectoid cementite serves as
the origin of fatigue cracks or brittle cracks.
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Furthermore, when a heating temperature is inadequate in
a reheating process for hot-rolling a bloom (slab) to be
rolled, the bloom (slab) is in a molten state partially,
cracks develop and, as a consequence, the bloom (slab)
breaks during hot rolling or cracks remain in the rail
after finish hot rolling, and therefore the product yield
deteriorates. What is more, another problem is that, in
some retention times at a reheating process,
decarburization is accelerated in the outer surface layer
of a bloom (slab), hardness lowers, caused by the
decrease of a carbon content in pearlite structures in
the outer surface layer of a rail after finish hot
rolling and, therefore, wear resistance at the head
portion of the rail deteriorates.
In view of the above situation, the developments of
rails have been promoted for solving the aforementioned
problems as shown below.
3) A rail wherein a eutectoid steel (containing 0.60
to 0.85% C) is used, the average size of block grains in
pearlite structures is made fine through rolling, and
thus ductility and toughness are enhanced (Japanese
Unexamined Patent Publication No. H8-109440).
4) A rail excellent in wear resistance wherein a
hyper-eutectoid steel (containing over 0.85 to 1.20% C)
is used, the density of cementite in lamella in pearlite
structures is increased, and, at the same time, hardness
is controlled (Japanese Unexamined Patent Publication No.
H8-246100).
5) A rail excellent in wear resistance wherein a
hyper-eutectoid steel (containing over 0.85 to 1.20% C)
is used, the density of cementite in lamella in pearlite
structures is increased, and, at the same time, hardness
is controlled by applying a heat treatment to the head
and/or web portion(s) (Japanese Unexamined Patent
Publication No. H9-137228).
6) A rail wherein a hyper-eutectoid steel
(containing over 0.85 to 1.20% C) is used, the average
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size of block grains in pearlite structures is made fine
through rolling and, thus, ductility and toughness are
enhanced (Japanese Unexamined Patent Publication No. H8-
109439).
In the rails proposed in the cases 3) and 4) above,
the wear resistance, ductility and toughness of pearlite
structures are enhanced by making the average size of
block grains in the pearlite structures fine, and the
wear resistance of the pearlite structures is further
enhanced by increasing a carbon content in a steel,
increasing the density of cementite in lamellae in the
pearlite structures and also increasing hardness.
However, despite the proposed technologies, the ductility
and toughness of rails have been insufficient in cold
regions where the temperature falls below the freezing
point. What is more, even when such average size of
block grains in pearlite structures as described above is
made still finer in an attempt to enhance the ductility
and toughness of rails, it has been difficult to
thoroughly suppress rail breakage in cold regions.
Further, in the rails proposed in the cases 4) and 5)
above, there is a problem in that, in some rolling
lengths and rolling end temperatures of rails, the
uniformity of the material quality of the rails in the
longitudinal direction and the ductility of the head
portions thereof cannot be secured. On top of that,
although it is possible to secure the hardness of
pearlite structures at head portions and suppress the
formation of pro-eutectoid cementite structures at web
portions by applying accelerated cooling to the head and
web portions of rails, it has still been difficult to
suppress the formation of pro-eutectoid cementite
structures, which serve as the starting points of fatigue
cracks and brittle cracks, at the base and base toe
portions of the rails, even when the heat treatment
methods disclosed above are employed. At a base toe
portion in particular, as the sectional area is smaller
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than those at head and web portions, the temperature of a
base toe portion at the end of rolling tends to be lower
than those of the other portions and, as a result, pro-
eutectoid cementite structures form before heat
treatment. Furthermore, at a web portion too, there are
still other problems in that: pro-eutectoid cementite
structures are likely to form because the segregation
bands of various alloying elements remain; and,
additionally, the temperature of the web portion is low
at the end of hot rolling. Therefore, an additional
problem has been that it is impossible to completely
prevent the fatigue cracks and brittle cracks originating
at base toe and web portions.
What is more, in the rail disclosed in the case 6)
above, though a technology of making the average size of
block grains in pearlite structures fine in a hyper-
eutectoid steel in an attempt to improve the ductility
and toughness of a rail is disclosed, it has been
difficult to thoroughly suppress the occurrence of rail
breakage in cold regions.
Disclosure of the Invention
In the aforementioned situation, a pearlitic steel
rail excellent in wear resistance and ductility and a
production method thereof are looked for, to make it
possible, in a rail of pearlite structure having a high
carbon content, to realize: a superior wear resistance at
the head portion of the rail; a high resistance to rail
breakage by enhancing ductility; the prevention of the
formation of pro-eutectoid cementite structures by
optimizing cooling conditions; and, in addition to those,
the uniformity in material characteristics in the
longitudinal direction of the rail and the suppression of
decarburization at the outer surface of the rail.
The present invention provides a pearlitic steel
rail excellent in wear resistance and ductility and a
production method thereof, wherein, in a rail used for a
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heavy load railway, the wear resistance and ductility
required of the railhead portion are enhanced, the
resistance to rail breakage is improved in particular,
and the fracture resistance of the web, base and base toe
portions of the rail is improved by preventing pro-
eutectoid cementite structures from forming.
Further, the present invention provides a high-
efficiency and high-quality pearlitic steel rail,
wherein: cracking and breakage during hot rolling are
prevented by optimizing the maximum heating temperature
and the retention time at a reheating process in the
event of hot-rolling a high-carbon steel bloom (slab) for
rail rolling; and, in addition, the deterioration of wear
resistance and fatigue strength is suppressed by
controlling decarburization in the outer surface layer of
the rail.
Still further, the present invention provides a
method for producing a pearlitic steel rail excellent in
wear resistance and ductility, wherein, in a rail having
a high carbon content, the occurrence of cracks caused by
fatigue, brittleness and lack of toughness is prevented
and, at the same time, the wear resistance of the head
portion, the uniformity in material quality in the
longitudinal direction of the rail and the ductility of
the head portion of the rail are secured by applying
accelerated cooling to the head, web and base portions of
the rail immediately after the end of hot rolling or
within a certain time period thereafter, further
optimizing the selection of an accelerated cooling rate
at the head portion, a rail length at rolling, and a
temperature at the end of rolling, and, by so doing,
suppressing the formation of pro-eutectoid cementite
structures.
The gist of the present invention, that attains the
above object, is described in the following items:
(1) A pearlitic steel rail excellent in wear
resistance and ductility, characterized in that, in a
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steel rail having pearlite structures containing, in
mass, 0.65 to 1.40% C, the number of the pearlite blocks
having grain sizes in the range from 1 to 15 lAm is 200 or
more per 0.2 mm2 of observation field at least in a part
of the region down to a depth of 10 mm from the surface
of the corners and top of the head portion.
(2) A pearlitic steel rail excellent in wear
resistance and ductility, characterized in that, in a
steel rail having pearlite structures containing, in
mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, and 0.05 to
2.00% Mn, the number of the pearlite blocks having grain
sizes in the range from 1 to 15 m is 200 or more per 0.2
mm2 of observation field at least in a part of the region
down to a depth of 10 mm from the surface of the corners
and top of the head portion.
(3) A pearlitic steel rail excellent in wear
resistance and ductility, characterized in that, in a
steel rail having pearlite structures containing, in
mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, 0.05 to 2.00%
Mn, and 0.05 to 2.00% Cr, the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m is
200 or more per 0.2 mm2 of observation field at least in
a part of the region down to a depth of 10 mm from the
surface of the corners and top of the head portion.
(4) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (3), characterized in that the C content of
the steel rail is over 0.85 to 1.40%.
(5) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (4), characterized in that the length of the
rail after hot rolling is 100 to 200 m.
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(6) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (5), characterized in that the hardness in
the region down to a depth of at least 20 mm from the
surface of the corners and top of the head portion is in
the range from 300 to 500 Hy.
(7) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (6), characterized by further containing, in
mass, 0.01 to 0.50% Mo.
(8) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (7), characterized by further containing, in
mass, one or more of 0.005 to 0.50% V, 0.002 to 0.050%
Nb, 0.0001 to 0.0050% B, 0.10 to 2.00% Co, 0.05 to 1.00%
Cu, 0.05 to 1.00% Ni, and 0.0040 to 0.0200% N.
(9) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (1) to (8), characterized by further containing, in
mass, one or more of 0.0050 to 0.0500% Ti, 0.0005 to
0.0200% Mg, 0.0005 to 0.0150% Ca, 0.0080 to 1.00% Al, and
0.0001 to 0.2000% Zr.
(10) A pearlitic steel rail excellent in wear
resistance and ductility according to any one of the
items (4) to (9), characterized by reducing the amount of
pro-eutectoid cementite structures forming in the web
portion of the rail so that the number of the pro-
eutectoid cementite network intersecting two line
segments each 300 m in length crossing each other at
right angles (the number of intersecting pro-eutectoid
cementite network, NC) at the center of the centerline in
the web portion of the rail may satisfy the expression NC
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CE in relation to the value of CE defined by the
following equation (1):
CE = 60([mass % C]) + 10([mass % Si]) + 10([mass %
Mn]) + 500([mass % P]) + 50([mass % S]) + 30([mass %
Cr]) + 50 .... (1).
(11) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility, characterized
by, in the hot rolling of a steel rail containing 0.65 to
1.40 mass % C: applying finish rolling so that the
temperature of the rail surface may be in the range from
850 C to 1,000 C and the sectional area reduction ratio
at the final pass may be 6% or more; then applying
accelerated cooling to the head portion of said rail at a
cooling rate in the range from 1 to 30 C/sec. from the
austenite temperature range to a temperature not higher
than 550 C; and controlling the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m so
as to be 200 or more per 0.2 mm2 of observation field at
least in a part of the region down to a depth of 10 mm
from the surface of the corners and top of the head
portion.
(12) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility, characterized
by, in the hot rolling of a steel rail containing, in
mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, and 0.05 to
2.00% Mn: applying finish rolling so that the temperature
of the rail surface may be in the range from 850 C to
1,000 C and the sectional area reduction ratio at the
final pass may be 6% or more; then applying accelerated
cooling to the head portion of said rail at a cooling
rate in the range from 1 to 30 C/sec. from the austenite
temperature range to a temperature not higher than 550 C;
and controlling the number of the pearlite blocks having
grain sizes in the range from 1 to 15 m so as to be 200
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or more per 0.2 mm2 of observation field at least in a
part of the region down to a depth of 10 mm from the
surface of the corners and top of the head portion.
(13) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility, characterized
by, in the hot rolling of a steel rail containing, in
mass, 0.65 to 1.40% C, 0.05 to 2.00% Si, 0.05 to 2.00%
Mn, and 0.05 to 2.00% Cr: applying finish rolling so that
the temperature of the rail surface may be in the range
from 850 C to 1,000 C and the sectional area reduction
ratio at the final pass may be 6% or more; then applying
accelerated cooling to the head portion of said rail at a
cooling rate in the range from 1 to 30 C/sec. from the
austenite temperature range to a temperature not higher
than 550 C; and controlling the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m so
as to be 200 or more per 0.2 mm2 of observation field at
least in a part of the region down to a depth of 10 mm
from the surface of the corners and top of the head
portion.
(14) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (13), characterized in that,
at the finish rolling in the hot rolling of said steel
rail, continuous finish rolling is applied so that two or
more rolling passes may be applied at a sectional area
reduction ratio of 1 to 30% per pass and the time period
between the passes may be 10 sec. or less.
(15) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (13), characterized by
applying accelerated cooling to the head portion of said
rail at a cooling rate in the range from 1 to 30 C/sec.
from the austenite temperature range to a temperature not
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higher than 550 C within 200 sec. after the end of the
finish rolling in the hot rolling of said steel rail.
(16) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (13), characterized by
applying accelerated cooling within 200 sec. after the
end of the finish rolling in the hot rolling of said
steel rail: to the head portion of said rail at a cooling
rate in the range from 1 to 30 C/sec. from the austenite
temperature range to a temperature not higher than 550 C;
and to the web and base portions of said rail at a
cooling rate in the range from 1 to 10 C/sec. from the
austenite temperature range to a temperature not higher
than 650 C.
(17) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by, in a
reheating process for a bloom or slab containing
aforementioned steel composition, reheating said bloom or
slab so that: the maximum heating temperature (Tmax, C)
of said bloom or slab may satisfy the expression Tmax
CT in relation to the value of CT defined by the
following equation (2) composed of the carbon content of
said bloom or slab; and the retention time (Mmax, min.)
of said bloom or slab after said bloom or slab is heated
to a temperature of 1,100 C or above may satisfy the
expression Mmax CM in relation to the value of CM
defined by the following equation (3) composed of the
carbon content of said bloom or slab:
CT - 1,500 - 140([mass % C]) - 80([mass % C])2
....
(2),
CM = 600 - 120([mass % C]) 60([mass % C])2
.... (3).
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(18) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by
applying accelerated cooling, after hot-rolling a bloom
or slab containing aforementioned steel composition into
the shape of a rail: within 60 sec. after the hot
rolling, to the base toe portions of said steel rail at a
cooling rate in the range from 5 to 20 C/sec. from the
austenite temperature range to a temperature not higher
than 650 C; and to the head, web and base portions of
said steel rail at a cooling rate in the range from 1 to
10 C/sec. from the austenite temperature range to a
temperature not higher than 650 C.
(19) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by
applying accelerated cooling, after hot-rolling a bloom
or slab containing aforementioned steel composition into
the shape of a rail: within 100 sec. after the hot
rolling, to the web portion of said steel rail at a
cooling rate in the range from 2 to 20 C/sec. from the
austenite temperature range to a temperature not higher
than 650 C; and to the head and base portions of said
steel rail at a cooling rate in the range from 1 to
10 C/sec. from the austenite temperature range to a
temperature not higher than 650 C.
(20) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by
applying accelerated cooling, after hot-rolling a bloom
or slab containing aforementioned steel composition into
the shape of a rail: within 60 sec. after the hot
rolling, to the base toe portions of said steel rail at a
cooling rate in the range from 5 to 20 C/sec. from the
austenite temperature range to a temperature not higher
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than 650 C; within 100 sec. after the hot rolling, to the
web portion of said steel rail at a cooling rate in the
range from 2 to 20 C/sec. from the austenite temperature
range to a temperature not higher than 650 C; and to the
head and base portions of said steel rail at a cooling
rate in the range from 1 to 10 C/sec. from the austenite
temperature range to a temperature not higher than 650 C.
(21) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by,
after hot-rolling a bloom or slab containing
aforementioned steel composition into the shape of a
rail: within 60 sec. after the hot rolling, raising the
temperature at the base toe portions of said steel rail
to a temperature 50 C to 100 C higher than the
temperature before the temperature rising; and also
applying accelerated cooling to the head, web and base
portions of said steel rail at a cooling rate in the
range from 1 to 10 C/sec. from the austenite temperature
range to a temperature not higher than 650 C.
(22) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by,
after hot-rolling a bloom or slab containing
aforementioned steel composition into the shape of a
rail: within 100 sec. after the hot rolling, raising the
temperature at the web portion of said steel rail to a
temperature 20 C to 100 C higher than the temperature
before the temperature rising; and also applying
accelerated cooling to the head, web and base portions of
said steel rail at a cooling rate in the range from 1 to
10 C/sec. from the austenite temperature range to a
temperature not higher than 650 C.
(23) A method for producing a pearlitic steel rail
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excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by,
after hot-rolling a bloom or slab containing
aforementioned steel composition into the shape of a
rail: within 60 sec. after the hot rolling, raising the
temperature at the base toe portions of said steel rail
to a temperature 20 C to 100 C higher than the
temperature before the temperature rising; within 100
sec. after the hot rolling, raising the temperature at
the web portion of said steel rail to a temperature 20 C
to 100 C higher than the temperature before the
temperature rising; and also applying accelerated cooling
to the head, web and base portions of said steel rail at
a cooling rate in the range from 1 to 10 C/sec. from the
austenite temperature range to a temperature not higher
than 650 C.
(24) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by, in
the event of acceleratedly cooling the head portion of
said steel rail from the austenite temperature range,
applying the accelerated cooling so that the cooling rate
(ICR, C/sec.) in the temperature range from 750 C to
650 C at a head inner portion 30 mm in depth from the
head top surface of said steel rail may satisfy the
expression ICR g OCR in relation to the value of OCR
defined by the following equation (4) composed of the
chemical compositions of said steel rail:
OCR = 0.6 + 10 x ([%C] - 0.9) - 5 x ([%C] - 0.9) x
[%Si] 0.17[%Mn] - 0.13[%Cr]
.... (4).
(25) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (16), characterized by, in
the event of acceleratedly cooling the head portion of
said steel rail from the austenite temperature range,
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applying the accelerated cooling so that the value of TCR
defined by the following equation (5) composed of the
respective cooling rates in the temperature range from
750 C to 500 C at the surfaces of the head top portion
(TH, C/sec.), the head side portions (TS, C/sec.) and
the lower chin portions (TJ, C/sec.) of said steel rail
may satisfy the expression 4CCR TCR 2CCR in relation
to the value of CCR defined by the following equation (4)
composed of the chemical compositions of said steel rail:
CCR = 0.6 + 10 x ([%C] - 0.9) - 5 x ([%C] - 0.9) x
[%Si] - 0.17[%Mn] - 0.13[%Cr] .... (4),
TCR = 0.05TH ( C/sec.) + 0.10TS ( C/sec.) + 0.50TJ
( C/sec.) .... (5).
(26) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (25), characterized in that
the C content of the steel rail is 0.85 to 1.40%.
(27) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (26), characterized in that
the length of the rail after hot rolling is 100 to 200 m.
(28) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (27), characterized in that
the hardness in the region down to a depth of at least 20
mm from the surface of the corners and top of the head
portion of a pearlitic steel rail according to any one of
the items (1) to (10) is in the range from 300 to 500 Hy.
(29) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (28), characterized in that
the steel rail further contains, in mass, 0.01 to 0.50%
Mo.
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(30) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (29), characterized in that
the steel rail further contains, in mass, one or more of
0.005 to 0.50% V, 0.002 to 0.050% Nb, 0.0001 to 0.0050%
B, 0.10 to 2.00% Co, 0.05 to 1.00% Cu, 0.05 to 1.00% Ni,
and 0.0040 to 0.0200% N.
(31) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (30), characterized in that
the steel rail further contains, in mass, one or more of
0.0050 to 0.0500% Ti, 0.0005 to 0.0200% Mg, 0.0005 to
0.0150% Ca, 0.0080 to 1.00% Al, and 0.0001 to 0.2000% Zr.
(32) A method for producing a pearlitic steel rail
excellent in wear resistance and ductility according to
any one of the items (11) to (31), characterized by
reducing the amount of pro-eutectoid cementite structures
forming in the web portion of the rail so that the number
of the pro-eutectoid cementite network intersecting two
line segments each 300 m in length crossing each other
at right angles (the number of intersecting pro-eutectoid
cementite network, NC) at the center of the centerline in
the web portion of the rail may satisfy the expression NC
CE in relation to the value of CE defined by the
following equation (1):
CE = 60([mass % C]) + 10([mass % Si]) + 10([mass %
Mn]) + 500([mass % P]) + 50([mass % S]) + 30([mass %
Cr]) + 50
(1).
Brief Description of the Drawings
Fig. 1 is an illustration showing the denominations
of different portions of a rail.
Fig. 2 is a schematic representation of the method
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of evaluating the formation of pro-eutectoid cementite
network.
Fig. 3 is an illustration showing, in a section, the
denominations of different positions on the surface of
the head portion of a pearlitic steel rail excellent in
wear resistance and ductility according to the present
invention and the region where wear resistance is
required.
Fig. 4 is an illustration showing an outline of a
Nishihara wear tester.
Fig. 5 is an illustration showing the position from
which a test piece for the wear test referred to in
Tables 1 and 2 is cut out.
Fig. 6 is an illustration showing the position from
which a test piece for the tensile test referred to in
Tables 1 and 2 is cut out.
Fig. 7 is a graph showing the relationship between
the carbon contents and the amounts of wear loss in the
wear test results of the steel rails according to the
present invention shown in Table 1 (reference numerals 1
to 12) and the comparative steel rails shown in Table 2
(reference numerals 13 to 22).
Fig. 8 is a graph showing the relationship between
the carbon contents and the total elongation values in
the tensile test results of the steel rails according to
the present invention shown in Table 1 (reference
numerals 1 to 12) and the comparative steel rails shown
in Table 2 (reference numerals 17 to 22).
Fig. 9 is an illustration showing an outline of a
rolling wear tester for a rail and a wheel.
Fig. 10 is an illustration showing different
portions at a railhead portion in detail.
Best Mode for Carrying out the Invention
The present invention is hereafter explained in
detail.
The present inventors studied, in the first place,
CA 02749503 2011-08-10
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the relationship between the occurrence of rail breakage
and the mechanical properties of pearlite structures. As
a result, it has been confirmed that the occurrence of
the rail breakage originating from the railhead portion
correlates well with ductility evaluated in a tensile
test rather than toughness evaluated in an impact test,
in which a loading speed is comparatively high, because
the loading speed imposed on the railhead portion by
contact with a wheel is comparatively low.
Then the present inventors re-examined the
relationship between ductility and the block size of
pearlite structures in a steel rail of pearlite
structures having a high carbon content. As a result, it
has been confirmed that, though the ductility of pearlite
structures tends to improve as the average size of block
grains in the pearlite structures decreases, the
ductility does not improve sufficiently with the mere
decrease in the average size of the block grains in a
region where the average size of the block grains is very
fine.
In view of this, the present inventors studied
dominating factor of the ductility of pearlite structures
in a region where the average size of the block grains in
pearlite structures was very fine. As a result, it has
been discovered that the ductility of pearlite structures
correlates not with the average block grain size but with
the number of the fine pearlite block grains having
certain grain sizes and that the ductility of pearlite
structures significantly improves by controlling the
number of the fine pearlite block grains having certain
grain sizes to a certain value or more in a given area of
a visual field.
On the basis of the above findings, the present
inventors have discovered that, in a steel rail of
pearlite structures having a high carbon content, both
the wear resistance and the ductility at the railhead
portion are improved simultaneously by controlling the
CA 02749503 2011-08-10
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number of the fine pearlite block grains having certain
grain sizes in the railhead portion.
That is, an object of the present invention is, in a
high-carbon containing rail for heavy load railways, to
enhance the wear resistance at the head portion thereof,
and, at the same time, to prevent the occurrence of
fracture such as breakage of the rail by improving
ductility through the control of the number of the fine
pearlite block grains having certain grain sizes.
Next, the reasons for regulating the conditions in
the present invention are hereafter explained in detail.
(1) Regulations for the size and the number of pearlite
block grains
Firstly, the reasons are explained for regulating
the size of pearlite block grains, the size being used
for regulating the number of the pearlite block grains,
in the range from 1 to 15 m.
A pearlite block having a grain size larger than 15
pm does not significantly contribute to improving the
ductility of fine pearlite structures. On the other
hand, though a pearlite block having a grain size smaller
than 1 pm contributes to improving the ductility of fine
pearlite structures, the contribution thereof is
insignificant. For those reasons, the size of pearlite
block grains, the size being used for regulating the
number of the pearlite block grains, is regulated in the
range from 1 to 15 pm.
Secondly, the reasons are explained for regulating
the number of the pearlite block grains having grain
sizes in the range from 1 to 15 pm to 200 or more per 0.2
mm2 of observation field.
When the number of the pearlite block grains having
grain sizes in the range from 1 to 15 pm is less than 200
per 0.2 mm2 of observation field, it becomes impossible
to improve the ductility of fine pearlite structures. No
CA 02749503 2011-08-10
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upper limit is particularly set forth with regard to the
number of the pearlite block grains having grain sizes in
the range from 1 to 15 m, but, from restrictions on the
rolling temperature during hot rolling and the cooling
conditions during heat treatment in rail production,
1,000 grains per 0.2 mm2 of observation field is the
upper limit, substantially.
Thirdly, the reasons are explained for specifying
that the region, in which the number of the pearlite
block grains having grain sizes in the range from 1 to 15
m is determined to be 200 or more per 0.2 mm2 of
observation field, is at least a part of the region down
to a depth of 10 mm from the surface of the corners and
top of a head portion.
The rail breakage that originates from a railhead
portion begins, basically, from the surface of the head
portion. For this reason, in order to prevent rail
breakage, it is necessary to enhance the ductility of the
surface layer of a railhead portion, namely, to increase
the number of the pearlite block grains having grain
sizes in the range from 1 to 15 m. As a result of
experimentally examining the correlation between the
ductility of the surface layer of a railhead portion and
the pearlite blocks in the surface layer thereof, it has
been clarified that the ductility of the surface layer of
a railhead portion correlates with the pearlite block
size in the region down to a depth of 10 mm from the
surface of the head top portion. In addition, as a
result of further examining the correlation between the
ductility of the surface layer of a railhead portion and
the pearlite blocks in the surface layer thereof, it has
been confirmed that the ductility of the surface layer of
the railhead portion is improved and, consequently, the
rail breakage is inhibited as long as a region where the
number of the pearlite block grains having grain sizes in
the range from 1 to 15 m is 200 or more exists at least
CA 02749503 2011-08-10
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in a part of the aforementioned region. The above
regulations are determined on the basis of the results
from the aforementioned examinations.
Here, the method of measuring the size of pearlite
block grains is described. Methods of measuring pearlite
block grains include (i) the modified curling etch
method, (ii) the etch pit method, and (iii) the electron
back-scatter diffraction pattern (EBSP) method wherein an
SEM is used. In the above examinations, since the size
of the pearlite block grains was fine, it was difficult
to confirm the size by the modified curling etch method
(i) or the etch pit method (ii), and, therefore, the EBSP
method (iii) was employed.
The conditions of the measurement are described
hereafter. The measurement of the size of pearlite block
grains followed the conditions and procedures described
in the items (ii) to (vii) below, and the number of the
pearlite block grains having grain sizes in the range
from 1 to 15 m per 0.2 mm2 of observation field was
counted. The measurement was done at least in two
observation fields at each of observation positions, the
number of the grains in each of the observation fields
was counted according to the following procedures, and
the average of the numbers of the grains in two or more
observation fields was used as the value representing an
observation position.
= Pearlite block measurement conditions
(i) SEM: a high-resolution scanning electron
microscope
(ii) Pre-treatment for measurement: polishing
of a machined surface with diamond abrasive of 1 pm and
then electrolytic polishing
(iii)Observation field: 400 pm x 500 pm
(observation area, 0.2 am2)
(iv) SEM beam diameter: 30 nm
(v) Measurement step (interval): 0.1 to 0.9 m
CA 02749503 2011-08-10
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(vi) Identification of a grain boundary: when
the difference in crystal orientations at two adjacent
measurement points is 15 or more, then the grain
boundary between the measurement points is identified as
a pearlite block grain boundary (large angle grain
boundary).
(vii)Grain size measurement: after measuring
the area of each of pearlite block grains, the radius of
each crystal grain is calculated assuming that the
pearlite block grain is round, then the diameter is
calculated from it, and the value thus obtained is used
as the size of the pearlite block grain.
(2) Chemical composition of a steel rail
The reasons are explained in detail for regulating
the chemical composition of a steel rail in the ranges
specified in the claims.
C is an element effective for accelerating pearlitic
transformation and securing wear resistance. If the
amount of C is 0.65% or less, then a sufficient hardness
of pearlite structures in a railhead portion cannot be
secured, in addition pro-eutectoid ferrite structures
form, therefore wear resistance deteriorates, and, as a
result, the service life of the rail is shortened. If
the amount of C exceeds 1.40%, on the other hand, then
pro-eutectoid cementite structures form in pearlite
structures at the surface layer and the inside of a
railhead and/or the density of cementite phases in the
pearlite structures increases, and thus the ductility of
the pearlite structures deteriorates. In addition, the
number of intersecting pro-eutectoid cementite network
(NC) in the web portion of a rail increases and the
toughness of the web portion deteriorates. For those
reasons, the amount of C is limited in the range from
0.65 to 1.40%. Note that, for enhancing wear resistance
still more, it is desirable to set the amount of C to
over 0.85% by which the density of cementite phases in
CA 02749503 2011-08-10
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pearlite structures can increase still more and thus wear
resistance can further be enhanced.
Si is a component indispensable as a deoxidizing
agent. Also, Si is an element that increases the
hardness (strength) of a railhead portion by the solid
solution hardening effect of Si in a ferrite phase in
pearlite structures and, at the same time, improves the
hardness and toughness of the rail by inhibiting the
formation of pro-eutectoid cementite structures.
However, if the content of Si is less than 0.05%, then
these effects are not expected sufficiently, and no
tangible improvement in hardness and toughness is
obtained. If the content of Si exceeds 2.00%, on the
other hand, then surface defects occur in a great deal
during hot rolling and/or weldability deteriorates caused
by the formation of oxides. Besides, in that case,
pearlite structures themselves become brittle, thus not
only the ductility of a rail deteriorates but also
surface damage such as spalling occurs and, therefore,
the service life of the rail shortens. For those
reasons, the amount of Si is limited in the range from
0.05 to 2.00%.
Mn is an element that enhances hardenability,
secures the hardness of pearlite structures by decreasing
the pearlite lamella spacing, and thus improves wear
resistance. However, if the content of Mn is less than
0.05%, then the effects are insignificant and it becomes
difficult to secure the wear resistance required of a
rail. If the content of Mn is more than 2.00%, on the
other hand, then hardenability is increased remarkably,
therefore martensite structures detrimental to wear
resistance and toughness tend to form, and segregation is
accelerated. What is more, in a high-carbon steel (C >
0.85%) in particular, pro-eutectoid cementite structures
form in the web and other portions, the number of
intersecting pro-eutectoid cementite network (NC)
increases in the web portion, and thus the toughness of a
CA 02749503 2011-08-10
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rail deteriorates. For those reasons, the amount of Mn
is limited in the range from 0.05 to 2.00%.
Note that, for inhibiting the formation of pro-
eutectoid cementite structures in the web portion of a
rail, it is necessary to regulate the addition amounts of
P and S. For that purpose, it is desirable to control
their addition amounts within the respective ranges
specified below for the following reasons.
P is an element that strengthens ferrite and
enhances the hardness of pearlite structures. However,
since P is an element that easily causes segregation, if
the content of P exceeds 0.030%, it also accelerates the
segregation of other elements and, as a result, the
formation of pro-eutectoid cementite structures in a web
portion is significantly accelerated. Consequently, the
number of intersecting pro-eutectoid cementite network
(NC) in the web portion of a rail increases and the
toughness of the web portion deteriorates. For those
reasons, the amount of P is limited to 0.030% or less.
S is an element that contributes to the acceleration
of pearlitic transformation by generating MnS and forming
Mn-depleted zone around the MnS and is effective for
enhancing the toughness of pearlite structures by making
the size of pearlite blocks fine as a result of the above
contribution. However, if the content of S exceeds
0.025%, the segregation of Mn is accelerated and, as a
result, the formation of pro-eutectoid cementite
structures in a web portion is violently accelerated.
Consequently, the number of intersecting pro-eutectoid
cementite network (NC) in the web portion of a rail
increases and the toughness of the web portion
deteriorates. For those reasons, the amount of S is
limited to 0.025% or less.
Further, the elements of Cr, Mo, V, Nb, B, CO, Cu,
Ni, Ti, Mg, Ca, Al and Zr may be added, as required, to a
steel rail having the chemical composition specified
above for the purposes of: enhancing wear resistance by
CA 02749503 2011-08-10
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strengthening pearlite structures; preventing the
deterioration of toughness by inhibiting the formation of
pro-eutectoid cementite structures; preventing the
softening and embrittlement of a weld heat-affected zone;
improving the ductility and toughness of pearlite
structures; strengthening pearlite structures; preventing
the formation of pro-eutectoid cementite structures; and
controlling the hardness distribution in the cross
sections of the head portion and the inside of a rail.
Among those elements, Cr and Mo secure the hardness
of pearlite structures by raising the equilibrium
transformation temperature of pearlite and, in
particular, by decreasing the pearlite lamella spacing.
V and Nb inhibit the growth of austenite grains by
forming carbides and nitrides during hot rolling and
subsequent cooling and, in addition, improve the
ductility and hardness of pearlite structures by
precipitation hardening. Further, they stably form
carbides and nitrides during reheating and thus prevent
the heat-affected zones of weld joints from softening. B
reduces the dependency of a pearlitic transformation
temperature on a cooling rate and uniformalizes the
hardness distribution in a railhead portion. Co and Cu
dissolve in ferrite in pearlite structures and thus
increase the hardness of the pearlite structures. Ni
prevents embrittlement caused by the addition of Cu
during hot rolling, increases the hardness of a pearlitic
steel at the same time, and, in addition, prevents the
heat-affected zones of weld joints from softening.
Ti makes the structure of a heat-affected zone fine
and prevents the embrittlement of a weld joint. Mg and
Ca make austenite grains fine during the rolling of a
rail, accelerate pearlitic transformation at the same
time, and improve the ductility of pearlite structures.
Al strengthens pearlite structures and suppresses the
formation of pro-eutectoid cementite structure by
shifting a eutectoid transformation temperature toward a
CA 02749503 2011-08-10
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higher temperature and, at the same time, a eutectoid
carbon concentration toward a higher carbon, and thus
enhances the wear resistance of a rail and prevents the
toughness thereof from deteriorating. Zr forms Zr02
inclusions, which serve as solidification nuclei in a
high-carbon steel rail, and thus increases an equi-axed
crystal grain ratio in a solidification structure. As a
result, it suppresses the formation of segregation bands
at the center portion of a casting and the formation of
pro-eutectoid cementite structures detrimental to the
toughness of a rail. The main object of N addition is to
enhance toughness by accelerating pearlitic
transformation originating from austenite grain
boundaries and making pearlite structures fine.
The reasons for regulating each of the
aforementioned chemical compositions are hereunder
explained in detail.
Cr is an element that contributes to the hardening
(strengthening) of pearlite structures by raising the
equilibrium transformation temperature of pearlite and
consequently making the pearlite structures fine, and, at
the same time, enhances the hardness (strength) of the
pearlite structures by strengthening cementite phases.
If the content of Cr is less than 0.05%, however, the
effects are insignificant and the effect of enhancing the
hardness of a steel rail does not show. If Cr is
excessively added in excess of 2.00%, on the other hand,
then hardenability increases, martensite structures form
in a great amount, and the toughness of a rail
deteriorates. In addition, segregation is accelerated,
the amount of pro-eutectoid cementite structures forming
in a web portion increases, consequently the number of
intersecting pro-eutectoid cementite network (NC)
increases, and therefore the toughness of the web portion
of a rail deteriorates. For those reasons, the amount of
Cr is limited in the range from 0.05 to 2.00%.
No, like Cr, is an element that contributes to the
CA 02749503 2011-08-10
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hardening (strengthening) of pearlite structures by
raising the equilibrium transformation temperature of
pearlite and consequently narrowing the space between
adjacent pearlite lamellae and enhances the hardness
(strength) of pearlite structures as a result. If the
content of Mo is less than 0.01%, however, the effects
are insignificant and the effect of enhancing the
hardness of a steel rail does not show at all. If Mo is
excessively added in excess of 0.50%, on the other hand,
then the transformation rate of pearlite structures is
lowered significantly, and martensite structures
detrimental to toughness are likely to form. For those
reasons, the addition amount of Mo is limited in the
range from 0.01 to 0.50%.
V is an element effective for: making austenite
grains fine by the pinning effect of V carbides and V
nitrides when heat treatment for heating a steel material
to a high temperature is applied; further enhancing the
hardness (strength) of pearlite structures by the
precipitation hardening of V carbides and V nitrides that
form during cooling after hot rolling; and, at the same
time, improving ductility. V is also an element
effective for preventing the heat-affected zone of a weld
joint from softening by forming V carbides and V nitrides
in a comparatively high temperature range at a heat-
affected zone reheated to a temperature in the range of
not higher than the Aci transformation temperature. If
the content of V is less than 0.005%, however, the
effects are not expected sufficiently and the enhancement
of the hardness of pearlite structures and the
improvement of the ductility thereof are not realized.
If V is added in excess of 0.500%, on the other hand,
then coarse V carbides and V nitrides form, and the
toughness and the resistance to internal fatigue damage
of a rail deteriorate. For those reasons, the amount of
V is limited in the range from 0.005 to 0.500%.
Nb, like V, is an element effective for: making
CA 02749503 2011-08-10
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austenite grains fine by the pinning effect of Nb
carbides and Nb nitrides when heat treatment for heating
a steel material to a high temperature is applied;
further enhancing the hardness (strength) of pearlite
structures by the precipitation hardening of Nb carbides
and Nb nitrides that form during cooling after hot
rolling; and, at the same time, improving ductility. Nb
is also an element effective for preventing the heat-
affected zone of a welded joint from softening by forming
Nb carbides and Nb nitrides stably in the temperature
range from a low temperature to a high temperature at a
heat-affected zone reheated to a temperature in the range
of not higher than the Ac, transformation temperature.
If the content of Nb is less than 0.002%, however, the
effects are not expected and the enhancement of the
hardness of pearlite structures and the improvement of
the ductility thereof are not realized. If Nb is added
in excess of 0.050%, on the other hand, then coarse Nb
carbides and Nb nitrides form, and the toughness and the
resistance to internal fatigue damage of a rail
deteriorate. For those reasons, the amount of Nb is
limited in the range from 0.002 to 0. 050%.
B is an element that suppresses the formation of
pro-eutectoid cementite by forming carbo-borides of iron,
uniformalizes the hardness distribution in a head portion
at the same time by lowering the dependency of a
pearlitic transformation temperature on a cooling rate,
prevents the deterioration of the toughness of a rail,
and extends the service life of the rail as a result. If
the content of B is less than 0.0001%, however, the
effects are insufficient and no improvement in the
hardness distribution in a railhead portion is realized.
If B is added in excess of 0.0050%, on the other hand,
then coarse carbo-borides of iron form, and ductility,
toughness and resistance to internal fatigue damage are
significantly deteriorated. For those reasons, the
amount of B is limited in the range from 0.0001 to
CA 02749503 2011-08-10
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0.0050%.
co is an element that dissolves in ferrite in
pearlite structures and enhances the hardness (strength)
of the pearlite structures by solid solution
strengthening. Co is also an element that improves
ductility by increasing the transformation energy of
pearlite and making pearlite structures fine. If the
content of Co is less than 0.10%, however, the effects
are not expected. If Co is added in excess of 2.00%, on
the other hand, then the ductility of ferrite phases
deteriorates significantly, spalling damage occurs at a
wheel rolling surface, and resistance to the surface
damage of a rail deteriorates. For those reasons, the
amount of Co is limited in the range from 0.10 to 2.00%.
Cu is an element that dissolves in ferrite in
pearlite structures and enhances the hardness (strength)
of the pearlite structures by solid solution
strengthening. If the content of Cu is less than 0.05%,
however, the effects are not expected. If Cu is added in
excess of 1.00%, on the other hand, then hardenability is
enhanced remarkably and, as a result, martensite
structures detrimental to toughness are likely to form.
In addition, in that case, the ductility of ferrite
phases is significantly lowered and therefore the
ductility of a rail deteriorates. For those reasons, the
amount of Cu is limited in the range from 0.05 to 1.00%.
Ni is an element that prevents embrittlement caused
by the addition of Cu during hot rolling and, at the same
time, hardens (strengthens) a pearlitic steel through
solid solution strengthening by dissolving in ferrite.
In addition, Ni is an element that, at a weld heat-
affected zone, precipitates as the fine grains of the
intermetallic compounds of Ni3Ti in combination with Ti
and inhibits the softening of the weld heat-affected zone
by precipitation strengthening. If the content of Ni is
less than 0.01%, however, the effects are very small. If
Ni is added in excess of 1.00%, on the other hand, the
CA 02749503 2011-08-10
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ductility of ferrite phases is lowered significantly,
spalling damage occurs at a wheel rolling surface, and
resistance to the surface damage of a rail deteriorates.
For those reasons, the amount of Ni is limited in the
range from 0.01 to 1.00%.
Ti is an element effective for preventing the
embrittlement of the heat-affected zone of a weld joint
by taking advantage of the fact that carbides and
nitrides of Ti having precipitated during the reheating
of the weld joint do not dissolve again and thus making
fine the structure of the heat-affected zone heated to a
temperature in the austenite temperature range. If the
content of Ti is less than 0.0050%, however, the effects
are insignificant. If Ti is added in excess of 0.0500%,
on the other hand, then coarse carbides and nitrides of
Ti form and the ductility, toughness and resistance to
internal fatigue damage of a rail deteriorate
significantly. For those reasons, the amount of Ti is
limited in the range from 0.0050 to 0.0500%.
mg is an element effective for improving the
ductility of pearlite structures by forming fine oxides
in combination with 0, S, Al and so on, suppressing the
growth of crystal grains during reheating for the rolling
of a rail, and thus making austenite grains fine. In
addition, MgO and MgS make MnS disperse in fine grains,
thus form Mn-depleted zone around the MnS, and contribute
to the progress of pearlitic transformation. Therefore,
Mg is an element effective for improving the ductility of
pearlite structures by making a pearlite block size fine.
If the content of Mg is less than 0.0005%, however, the
effects are insignificant. If Mg is added in excess of
0.0200%, on the other hand, then coarse oxides of Mg form
and the toughness and resistance to internal fatigue
damage of a rail deteriorate. For those reasons, the
amount of Mg is limited in the range from 0.0005 to
0.0200%.
Ca has a strong bonding power with S and forms
CA 02749503 2011-08-10
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sulfides in the form of CaS. Further, CaS makes MnS
disperse in fine grains and thus forms Mn-depleted zone
around the MnS. Therefore, Ca contributes to the
progress of pearlitic transformation and, as a result, is
an element effective for improving the ductility of
pearlite structures by making a pearlite block size fine.
If the content of Ca is less than 0.0005%, however, the
effects are insignificant. If Ca is added in excess of
0.0150%, on the other hand, then coarse oxides of Ca form
and the toughness and resistance to internal fatigue
damage of a rail deteriorate. For those reasons, the
amount of Ca is limited in the range from 0.0005 to
0.0150%.
Al is an element that shifts a eutectoid
transformation temperature toward a higher temperature
and, at the same time, a eutectoid carbon concentration
toward a higher carbon. Thus, Al is an element that
strengthens pearlite structures and prevents the
deterioration of toughness, by inhibiting the formation
of pro-eutectoid cementite structures. If the content of
Al is less than 0.0080%, however, the effects are
insignificant. If Al is added in excess of 1.00%, on the
other hand, it becomes difficult to make Al dissolve in a
steel, thus coarse alumina inclusion serving as the
origins of fatigue damage form, and consequently the
toughness and resistance to internal fatigue damage of a
rail deteriorate. In addition, in that case, oxides form
during welding and weldability is remarkably
deteriorated. For those reasons, the amount of Al is
limited in the range from 0.0080 to 1.00%.
Zr is an element that functions as the
solidification nuclei in a high-carbon steel rail in
which y-Fe is the primary crystal of solidification,
because Zr02 inclusions have good lattice coherent with
y-Fe, thus increases an equi-axed crystal ratio in a
solidification structure, by so doing, inhibits the
CA 02749503 2011-08-10
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formation of segregation bands at the center portion of a
casting, and suppresses the formation of pro-eutectoid
cementite structures detrimental to the toughness of a
rail. If the amount of Zr is less than 0.0001%, however,
then the number of ZrO2 inclusions is so small that their
function as the solidification nuclei does not bear a
tangible effect, and, as a consequence, the effect of
suppressing the formation of pro-eutectoid cementite
structures is reduced. If the amount of Zr exceeds
0.2000%, on the other hand, then coarse Zr inclusions
form in a great amount, thus the toughness of a rail
deteriorates, internal fatigue damage originating from
coarse Zr system inclusions is likely to occur, and, as a
result, the service life of the rail shortens. For those
reasons, the amount of Zr is limited in the range from
0.0001 to 0.2000%.
N accelerates the pearlitic transformation
originating from austenite grain boundaries by
segregating at the austenite grain boundaries, and thus
makes the pearlite block size fine. Therefore, N is an
element effective for enhancing the toughness and
ductility of pearlite structures: If the content of N is
less than 0.0040%, however, the effects are
insignificant. If N is added in excess of 0.0200%, on
the other hand, it becomes difficult to make N dissolve
in a steel and gas holes functioning as the origins of
fatigue damage form in the inside of a rail. For those
reasons, the amount of N is limited in the range from
0.0040 to 0.0200%.
A steel rail that has such chemical composition as
described above is melted and refined in a commonly used
melting furnace such as a converter or an electric arc
furnace, then resulting molten steel is processed through
ingot casting and breakdown rolling or continuous
casting, and thereafter the resulting casting is produced
into rails through hot rolling. Subsequently,
accelerated cooling is applied to the head portion of a
CA 02749503 2011-08-10
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hot-rolled rail maintaining the high temperature heat at
the hot rolling or being reheated to a high temperature
for the purpose of heat treatment, and, by so doing,
pearlite structures having a high hardness can be stably
formed in the railhead portion.
As a method for controlling the number of the
pearlite blocks having grain sizes in the range from 1 to
p.m so as to be 200 or more per 0.2 mm2 of observation
field at least in a part of the region down to a depth of
10 10 mm from the surface of the corners and top of a
railhead portion in the above production processes, a
method desirable satisfies the conditions of: setting the
temperature during hot rolling as low as possible;
applying accelerated cooling as quickly as possible after
15 the rolling; by so doing, suppressing the growth of
austenite grains immediately after rolling; and raising
an area reduction ratio at the final rolling so that the
accelerated cooling may be applied while high strain
energy is accumulated in the austenite grains. Desirable
hot rolling and heat treatment conditions are as follows:
a final rolling temperature is 980 C or lower; an area
reduction ratio at the final rolling is 6% or more; and
an accelerated cooling rate is 1 C/sec. or more in
average of range from the austenite temperature range to
550 C.
Further, in the case where a rail is reheated for
the purpose of heat treatment, as it is impossible to
make use of the effect of strain energy, it is desirable
to set a reheating temperature as low as possible and an
accelerated cooling rate as high as possible. Desirable
conditions of heat treatment for reheating are as
follows: a reheating temperature is 1,000 C or lower; and
an accelerated cooling rate is 5 C/sec. or more in
average of range from the austenite temperature range to
550 C.
(3) Hardness of a railhead portion and the range of the
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hardness
Here, the reasons are explained for regulating the
hardness in the region down to a depth of 20 mm from the
surface of the corners and top of a railhead portion so
as to be in the range from 300 to 500 Hy.
In a steel haying chemical composition according to
the present invention, if hardness is below 300 Hy, then
it becomes difficult to secure a good wear resistance and
the service life of a rail shortens. If hardness exceeds
500 Hy, on the other hand, resistance to surface damage
is significantly deteriorated as a result of: the
accumulation of fatigue damage at a wheel rolling surface
caused by an extravagant improve in wear resistance;
and/or the occurrence of rolling fatigue damage such as
dark spot damage caused by the development of a
crystallographic texture. For those reasons, the
hardness of pearlite structures is limited in the range
from 300 to 500 in Hy.
Next, the reasons are explained for regulating the
portion, where the hardness is regulated in the range
from 300 to 500 Hy, so as to be in the region down to a
depth of 20 mm from the surface of the corners and top of
a head portion.
If the depth of the portion where the hardness is
regulated in the range from 300 to 500 Hy is less than 20
mm, then, in consideration of the service life of a rail,
the depth of the portion where the wear resistance
required of a rail must be secured is insufficient and it
becomes difficult to secure a sufficiently long service
life of the rail. If the portion where the hardness is
regulated in the range from 300 to 500 Hy extends down to
a depth of 30 mm or more from the surface of the corners
and top of a head portion, the rail service life is
further extended, which is more desirable.
In relation to the above, Fig. 1 shows the
denominations of different portions of a rail, wherein:
the reference numeral 1 indicates the head top portion,
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the reference numeral 2 the head side portions (corners)
at the right and left sides of the rail, the reference
numeral 3 the lower chin portions at the right and left
sides of the rail, and the reference numeral 4 the head
inner portion, which is located in the vicinity of the
position at a depth of 30 mm from the surface of the head
top portion in the center of the width of the rail.
Fig. 3 shows the denominations of different
positions of the surface of a head portion and the region
where the pearlite structures having the hardness of 300
to 500 Hv are required in a cross section of the head
portion of a pearlitic steel rail excellent in wear
resistance and ductility according to the present
invention. In the railhead portion, the reference
numeral 1 indicates the head top portion and the
reference numeral 2 the head corner portions, one of the
two head corner portions 2 being the gauge corner (G.C.)
portion that mainly contacts with wheels. The wear
resistance of a rail can be secured as long as the
pearlite structures having chemical composition according
to the present invention and having the hardness of 300
to 500 Hv are formed at least in the region shaded with
oblique lines in the figure.
Therefore, it is desirable that pearlite structures
having hardness controlled within the above range are
located in the vicinity of the surface of a railhead
portion that mainly contacts with wheels, and the other
portions may consist of any metallographic structures
other than a pearlite structure.
Next, the present inventors quantified the amount of
pro-eutectoid cementite structures forming in the web
portion of a rail. As a result of measuring the number
of the pro-eutectoid cementite network intersecting two
line segments of a prescribed length crossing each other
at right angles (hereinafter referred to as the number of
intersecting pro-eutectoid cementite network, NC) in an
observation field under a prescribed magnification, a
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good correlation has been found between the number of
intersecting pro-eutectoid cementite network and the
state of cementite structure formation, and it has been
clarified that the state of pro-eutectoid cementite
structure formation can be quantified on the basis of the
correlation.
Subsequently, the present inventors investigated the
relationship between the toughness of a web portion and
the state of pro-eutectoid cementite structure formation
using steel rails of pearlite structures having a high
carbon content. As a result, it has been clarified that,
in a steel rail of pearlite structures having a high
carbon content: (i) the toughness of the web portion of
the rail is in negative correlation with the number of
intersecting pro-eutectoid cementite network (NC); (ii)
if the number of intersecting pro-eutectoid cementite
network (NC) is not more than a certain value, then the
toughness of the web portion does not deteriorate; and
(iii) the threshold value of the number of intersecting
pro-eutectoid cementite network (NC) beyond which the
toughness deteriorates correlates with the chemical
compositions of the steel rail.
On the basis of the above findings, the present
inventors tried to clarify the relationship between the
threshold value of the number of intersecting pro-
eutectoid cementite network (NC) beyond which the
toughness of the web portion of a rail deteriorated, and
the chemical compositions of the steel rail, by using
multiple correlation analysis. As a result, it has been
found that the threshold value of the number of
intersecting pro-eutectoid cementite network (NC) beyond
which the toughness of a web portion decreases can be
defined by the value (CE) calculated from the following
equation (1) that evaluates the contributions of chemical
compositions (in mass %) in a steel rail.
Further, the present inventors studied a means for
improving the toughness of the web portion of a rail. As
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a result, it has been found that the amount of pro-
eutectoid cementite structures forming in the web portion
of a rail is reduced to a level lower than that of a
presently used steel rail and the toughness of the web
portion of the rail is prevented from deteriorating by
controlling the number of intersecting pro-eutectoid
cementite network (NC) in the web portion of the rail so
as to be not more than the value of CE calculated from
the chemical composition of the rail:
CE = 60[mass % C] 10 [mass % Si]
+ 10[mass % Mn] +
500[mass % P] + 50[mass % S] + 30[mass % Cr] - 54
....
(1),
NC (number of intersecting pro-eutectoid cementite
network in a web portion) CE (value of the
equation (1)).
Note that, in the present invention, in order to
reduce the number of intersecting pro-eutectoid cementite
network (NC) at the center of the centerline in the web
portion of a rail, it is effective: with regard to
continuous casting, (i) to optimize the soft reduction by
a means such as the control of a casting speed and (ii)
to make a solidification structure fine by lowering the
temperature of casting; and, with regard to the heat
treatment of a rail, (iii) to apply accelerated cooling
to the web portion of a rail in addition to the head
portion thereof. In order to reduce the number of
intersecting pro-eutectoid cementite network (NC) still
further, it is effective: to combine the above measures
in continuous casting and heat treatment; to add Al,
which has an effect of suppressing the formation of pro-
eutectoid cementite structures; and/or to add Zr, which
makes a solidification structure fine.
(4) Method for exposing pro-eutectoid cementite
structures in the web portion of a rail
The method for exposing pro-eutectoid cementite
structures described in the items 10 and 32 is explained
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hereunder. Firstly, a cross-sectional surface of the web
portion of a rail is polished with diamond abrasive,
subsequently, the polished surface is immersed in a
solution of picric acid and caustic soda, and thus pro-
eutectoid cementite structures are exposed. Some
adjustments may be required of the exposing conditions in
accordance with the condition of a polished surface, but,
basically, desirable exposing conditions are: an
immersion solution temperature is 80 C; and an immersion
time is approximately 120 min.
(5) Method for measuring the number of intersecting pro-
eutectoid cementite network (NC)
Next, the method for measuring the number of
intersecting pro-eutectoid cementite network (NC) is
explained. Pro-eutectoid cementite is likely to form at
the boundaries of prior austenite crystal grains. The
portion where pro-eutectoid cementite structures are
exposed at the center of the centerline on a sectional
surface of the web portion of a rail is observed with an
optical microscope. Then, the number of intersections
(expressed in the round marks in Fig. 2) of pro-eutectoid
cementite network with two line segments each 300 im in
length crossing each other at right angles is counted
under a magnification of 200. Fig. 2 schematically shows
the measurement method. The number of the intersecting
pro-eutectoid cementite network is defined as the total
of the intersections on the two line segments X and Y
each 300 lim in length crossing each other at right
angles, namely, [Xn = 4] + [Yn = 7]. Note that, in
consideration of uneven distribution of pro-eutectoid
cementite structures caused by the variation of the
intensity of segregation, it is desirable to carry out
the counting, at least, at 5 or more observation fields
and use the average of the counts as the representative
figure of the specimen.
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(6) Equation for calculating the value of CE
Here, the reason is explained for defining the
equation for calculating the value of CE as described
earlier. The equation for calculating the value of CE
has been obtained, using steel rails of pearlite
structures having a high carbon content, by taking the
procedures of: investigating the relationship between the
toughness of a web portion and the state of pro-eutectoid
cementite structure formation; and then clarifying the
relationship between the threshold value of the number of
intersecting pro-eutectoid cementite network (NC) beyond
which the toughness of the web portion deteriorates and
the chemical composition (in mass %) of the steel rail by
using multiple correlation analysis. The resulting
correlation equation (1) is shown below:
CE = 60[mass % C] - 10[mass % Si] + 10 [mass % Mn] +
500[mass % P] + 50[mass % S] + 30[mass % Cr] - 54
....
(1).
The coefficient affixed to the content of each of
the constituent chemical composition represents the
contribution of the relevant component to the formation
of cementite structures in the web portion of a rail, and
the sign + means that the relevant component has a
positive correlation with the formation of cementite
structures, and the sign - a negative correlation. The
absolute value of each of the coefficients represents the
magnitude of the contribution. A value of CE is defined
as an integer of the value calculated from the equation
above, round up numbers of five and above and drop
anything under five. Note that, in some combinations of
the chemical composition specified in the above equation,
the value of CE may be 0 or negative. Such a case that
the value of CE is 0 or negative is regarded as outside
of the scope of the present invention, even if the
contents of the chemical composition conform to the
relevant ranges specified earlier.
CA 02749503 2011-08-10
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In addition, the present inventors examined the
causes for generating cracks in a bloom (slab) having a
high carbon content in the processes of reheating and hot
rolling the casting into rails. As a result, it has been
clarified that: some parts of a casting are melted at
segregated portions in solidification structures in the
vicinity of the outer surface of the casting where the
heating temperature of the casting is the highest; the
melted parts burst by the subsequent rolling; and thus
cracks are generated. It has also been clarified that,
the higher the maximum heating temperature of a casting
is or the higher the carbon content of a casting is, the
more the cracks tend to be generated.
On the basis of the above findings, the present
inventors experimentally studied the relationship between
the maximum heating temperature of a casting at which
melted parts that caused cracks were generated and the
carbon content in the casting. As a result, it has been
found that the maximum heating temperature of a casting
at which the melted parts are generated can be regulated
by a quadratic expression which is shown as the following
equation (2) composed of the carbon content (in mass %)
of the casting, and that the melted parts of a casting in
a reheated state and accompanying cracks or breaks during
hot rolling can be prevented by controlling the maximum
heating temperature (Tmax, C) of the casting to not more
than the value of CT calculated from the quadratic
equation:
CT = 1500 - 140([mass % C]) - 80([mass % C])2
.... (2).
Next, the present inventors analyzed the factors
that accelerated the decarburization in the outer surface
layer of the bloom (slab) having a high carbon content in
a reheating process for hot rolling the bloom (slab) into
rails. As a result, it has been clarified that the
decarburization in the outer surface layer of the bloom
(slab) is significantly influenced by a temperature and a
CA 02749503 2011-08-10
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retention time in the reheating of the casting and
moreover the carbon content in the bloom (slab).
On the basis of the above findings, the present
inventors studied the relationship among a temperature
and a retention time in the reheating of the bloom
(slab), a carbon content in the bloom (slab), and the
amount of decarburization in the outer surface layer of
the bloom (slab). As a result, it has been found that,
the longer the retention time at a temperature not lower
than a certain temperature is and the higher the carbon
content in the bloom (slab) is, the more the
decarburization in the outer surface layer of the bloom
(slab) is accelerated.
In addition, the present inventors experimentally
studied the relationship between the carbon content in
the bloom (slab) and a retention time in the reheating of
the bloom (slab) that does not cause the deterioration of
the properties of a rail after final rolling. As a
result, it has been found that, when a reheating
temperature is 1,100 C or higher, the retention time of
the bloom (slab) can be regulated by a quadratic
expression which is shown as the following equation (3)
composed of the carbon content (in mass %) of the bloom
(slab), and that the decrease of the carbon content and
the deterioration of hardness in pearlite structures in
the outer surface layer of the bloom (slab) can be
suppressed and also the deterioration of the wear
resistance and the fatigue strength of a rail after final
rolling can be suppressed by controlling the reheating
time of the bloom (slab) (Mmax, min.) to not more than
the value of CM calculated from the quadratic equation:
CM = 600 - 120([mass % C]) - 60([mass % C))2
....
(3).
As stated above, the present inventors have found
that, by optimizing the maximum heating temperature of
the bloom (slab) having a high carbon content and the
retention time thereof at a heating temperature not
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lower than a certain temperature in a reheating process
for hot rolling the bloom (slab) into rails: the partial
melting of the bloom (slab) is prevented and thus cracks
and breaks are prevented during hot rolling; further the
decarburization in the outer surface layer of a rail is
inhibited and thus the deterioration of wear resistance
and fatigue strength is suppressed; and, as a
consequence, a high quality rail can be produced
efficiently.
In other words, the present invention makes it
possible to efficiently produce a high quality rail by
preventing the partial melting of the bloom (slab) having
a high carbon content and suppressing the decarburization
in the outer surface layer of the bloom (slab) in a
reheating process for hot rolling the bloom (slab) into
rails. The conditions specified in the present invention
are explained hereunder.
(7) Reasons for limiting the maximum heating temperature
(Tmax, C) of a bloom (slab) in a reheating process for
hot rolling
Here, the reasons are explained in detail for
limiting the maximum heating temperature (Tmax, C) of a
bloom (slab) to not more than the value of CT calculated
from the carbon content of a steel rail in a reheating
process for hot rolling the bloom (slab) into rails.
The present inventors experimentally investigated
the factors that caused partial melting to occur in a
bloom (slab) having a high carbon content in a reheating
process for hot rolling the bloom (slab) into rails and
thus cracks to be generated in the bloom (slab) during
hot rolling. As a result, it has been confirmed that,
the higher the maximum heating temperature of a bloom
(slab) is and the higher the carbon content thereof is,
partial melting is apt to occur in the bloom (slab)
during reheating and cracks are apt to be generated
during hot rolling.
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On the basis of the findings, the present inventors
tried to find the relationship between the carbon content
of a bloom (slab) and the maximum heating temperature
thereof beyond which partial melting occurred in the
bloom (slab) by using multiple correlation analysis. The
resulting correlation equation (2) is shown below:
CT = 1500 - 140([mass % C]) - 80([mass % C])2
....
(2).
As stated above, the equation (2) is an experimental
regression equation, and partial melting in a bloom
(slab) during reheating and accompanying cracks and
breaks during rolling can be prevented by controlling the
maximum heating temperature (Tmax, C) of the bloom
(slab) to not more than the value of CT calculated from
the quadratic equation composed of the carbon content of
the bloom (slab).
(8) Reasons for limiting the retention time (Mmax, min.)
of a bloom (slab) in a reheating process for hot rolling
Here, the reasons are explained in detail for
limiting the retention time (Mmax, min.) of a bloom
(slab) heated to a temperature of 1,100 C or higher in a
reheating process for hot rolling the bloom (slab) into
rails to not more than the value of CM calculated from
the carbon content of a steel rail.
The present inventors experimentally investigated
the factors that increased the amount of decarburization
in the outer surface layer of a bloom (slab) having a
high carbon content in a reheating process for hot
rolling the bloom (slab) into rails. As a result, it has
been clarified that, the longer the retention time at a
temperature not lower than a certain temperature is and
the higher the carbon content in a bloom (slab) is, the
more the decarburization is accelerated during reheating.
On the basis of the findings, the present inventors
tried to find out the relationship, in the reheating
temperature range of 1,100 C or higher where the
CA 02749503 2011-08-10
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decarburization of a casting was significant, between the
carbon content of a bloom (slab) and the retention time
of the bloom (slab) beyond which the properties of a rail
after final rolling deteriorated by using multiple
correlation analysis. The resulting correlation equation
(3) is shown below:
CM = 600 - 120([mass % C]) - 60([mass % C])2
....
(3) .
As stated above, the equation (3) is an experimental
regression equation, and the decrease in the carbon
content and the hardness of pearlite structures in the
outer surface layer of a bloom (slab) is inhibited and
thus the deterioration of the wear resistance and the
fatigue strength of a rail after final rolling is
suppressed by controlling the retention time (Mmax, min.)
of the bloom (slab) in the reheating temperature range of
1,100 C or higher to not more than the value of CM
calculated from the quadratic equation.
Note that no lower limit is particularly specified
for a retention time (Mmax, min.) in the reheating of a
bloom (slab), but it is desirable to control a retention
time to 250 min. or longer from the viewpoint of heating
a casting sufficiently and uniformly and securing
formability at the time of the rolling of a rail.
with regard to the control of the temperature and
the time of reheating as specified above in a reheating
process for hot rolling a bloom (slab) into rails, it is
desirable. to directly measure a temperature at the outer
surface of a bloom (slab) and to control the temperature
thus obtained and the time. However, when the
measurement is difficult industrially, by controlling the
average temperature of the atmosphere in a reheating
furnace and the resident time in the furnace in a
prescribed temperature range of the furnace atmosphere
too, similar effects can be obtained and a high-quality
rail can be produced efficiently.
Next, the present inventors studied a heat treatment
CA 02749503 2011-08-10
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method capable of, in a steel rail having a high carbon
content, enhancing the hardness of pearlite structures in
the railhead portion and suppressing the formation of
pro-eutectoid cementite structures in the web and base
portions thereof. As a result, it has been confirmed
that, with regard to a rail after hot rolling, it is
possible to enhance the hardness of the railhead portion
and suppress the formation of pro-eutectoid cementite
structures in the web and base portions thereof by
applying accelerated cooling to the head portion and also
another accelerated cooling to the web and base portions
either from the austenite temperature range within a
prescribed time after rolling or after the rail is heated
again to a certain temperature.
As the first step of the above studies, the present
inventors studied a method for hardening pearlite
structures in a railhead portion in commercial rail
production. As a result, it has been found that: the
hardness of pearlite structures in a railhead portion
correlates with the time period from the end of hot
rolling to the beginning of the subsequent accelerated
cooling and the rate of the accelerated cooling; and it
is possible to form pearlite structures in a railhead
portion and harden the portion by controlling the time
period after the end of hot rolling and the rate of
subsequent accelerated cooling within respective
prescribed ranges and further by controlling the
temperature at the end of the accelerated cooling to not
lower than a prescribed temperature.
As the second step, the present inventors studied a
method that makes it possible to suppress the formation
of pro-eutectoid cementite structures in the web and base
portions of a rail in commercial rail production. As a
result, it has been found that: the formation of pro-
eutectoid cementite structures correlates with the time
period from the end of hot rolling to the beginning of
the subsequent accelerated cooling and the conditions of
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the accelerated cooling; and it is possible to suppress
the formation of pro-eutectoid cementite structures by
controlling the time period after the end of hot rolling
within a prescribed range and further by either (i)
controlling the accelerated cooling rate within a
prescribed range and the accelerated cooling end
temperature to not lower than a prescribed temperature,
or (ii) applying heating up to a temperature within a
prescribed temperature range and thereafter controlling
the accelerated cooling rate within a prescribed range.
In addition to the above production methods, the
present inventors studied a rail production method for
securing the uniformity of the material quality of a rail
in the longitudinal direction in the above production
methods. As a result, it has been clarified that, when
the length of a rail at hot rolling exceeds a certain
length: the temperature difference between the two ends
of the rail and the middle portion thereof and moreover
between the ends of the rail after the rolling is
excessive; and, by the above-mentioned rail production
method, it is difficult to control the temperature and
the cooling rate over the whole length of the rail and
thus the material quality of the rail in the longitudinal
direction becomes uneven. Then, the present inventors
studied an optimum rolling length of a rail for securing
the uniformity of the material quality of the rail
through the test rolling of real rails. As a result, it
has been found that a certain adequate range exists in
the rolling length of a rail in consideration of
economical efficiency.
In addition, the present inventors studied a rail
production method for securing the ductility of a
railhead portion. As a result, it has been found that:
the ductility of a railhead portion correlates with the
temperature and the area reduction ratio of hot rolling,
the time period between rolling passes and the time
period from the end of final rolling to the beginning of
CA 02749503 2011-08-10
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heat treatment; and it is possible to secure both the
ductility of a railhead portion and the formability of a
rail at the same time by controlling the temperature of
the railhead portion at final rolling, the area reduction
ratio, the time period between rolling passes and the
time period to the beginning of heat treatment within
respective prescribed ranges.
As stated above, in the present invention, it has
been found that, with regard to a steel rail having a
high carbon content: it is possible to harden the
railhead portion and thus secure the wear resistance of
the railhead portion and to suppress the formation of
pro-eutectoid cementite structures at the web and base
portions of the rail, the structures being detrimental to
the fatigue cracking and brittle fracture, by applying
accelerated cooling to the head, web and base portions of
the rail within a prescribed time period after the end of
hot rolling and, in addition, by applying another
accelerated cooling to the web and base toe portions of
the rail after the rail is heated; and further it is
possible to secure the wear resistance of the railhead
portion, the uniformity of the material quality of the
rail in the longitudinal direction, the ductility of the
railhead portion, and the fatigue strength and fracture
toughness of the web and base portions of the rail by
optimizing the length of the rail at rolling, the
temperature of the railhead portion at final rolling, the
area reduction ratio, the time period between rolling
passes, and the time period from the end of rolling to
the beginning of heat treatment.
In other words, the present invention makes it
possible to, in a steel rail having a high carbon
content: make the size of pearlite blocks fine; secure
the ductility of the railhead portion; prevent the
deterioration of the wear resistance of the railhead
portion and the fatigue strength and fracture toughness
of the web and base portions of the rail; and secure the
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uniformity of the material quality of the rail in the
longitudinal direction.
(9) Reasons for limiting the conditions of accelerated
cooling
Here, the reasons are explained in detail for
limiting the time period from the end of hot rolling to
the beginning of accelerated cooling, and the rate and
the temperature range of accelerated cooling in the
items 11 to 16.
In the first place, explanations are given regarding
the time period from the end of hot rolling to the
beginning of accelerated cooling.
When the time period from the end of hot rolling to
the beginning of accelerated cooling exceeds 200 sec.,
with the chemical composition according to the present
invention, austenite grains coarsen after rolling, as a
consequence pearlite blocks coarsen, and ductility is not
improved sufficiently, and, with some chemical
composition according to the present invention, pro-
eutectoid cementite structures form and the fatigue
strength and toughness of a rail deteriorate. For those
reasons, the time period from the end of hot rolling to
the beginning of accelerated cooling is limited to not
longer than 200 sec. Note that, even if the time period
exceeds 200 sec., the material quality of a rail is not
significantly deteriorated except for ductility.
Therefore, as far as the time period is not longer than
250 sec., a rail quality acceptable for actual use can be
secured.
Meanwhile, in a section of a rail immediately after
the end of hot rolling, an uneven temperature
distribution exists caused by heat removal by rolling
rolls during rolling and so on, and, as a result,
material quality in the rail section becomes uneven after
accelerated cooling. In order to suppress temperature
unevenness in a rail section and uniformalize material
CA 02749503 2011-08-10
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quality in the rail section, it is desirable to begin
accelerated cooling after the lapse of not less than 5
sec. from the end of the rolling.
Next, explanations are given regarding the range of
an accelerated cooling rate.
First, the conditions of accelerated cooling at a
railhead portion are explained. When the accelerated
cooling rate of a railhead portion is below 1 C/sec.,
with the chemical composition according to the present
invention, the railhead portion cannot be hardened and it
becomes difficult to secure the wear resistance of the
railhead portion. In addition, pro-eutectoid cementite
structures form and the ductility of the rail
deteriorates. What is more, the pearlitic transformation
temperature rises, pearlite blocks coarsen, and the
ductility of the rail deteriorates. When an accelerated
cooling rate exceeds 30 C/sec., on the other hand, with
the chemical composition according to the present
invention, martensite structures form and the toughness
of a railhead portion deteriorates significantly. For
those reasons, the accelerated cooling rate of a railhead
portion is limited in the range from 1 to 30 C/sec.
Note that the accelerated cooling rate mentioned
above is not a cooling rate during cooling but an average
cooling rate from the beginning to the end of accelerated
cooling. Therefore, as far as an average cooling rate
from the beginning to the end of accelerated cooling is
within the range specified above, it is possible to make
a pearlite block size fine and simultaneously harden a
railhead portion.
Next, explanations are given regarding the
temperature range of accelerated cooling. When
accelerated cooling at a railhead portion is finished at
a temperature above 550 C, an excessive thermal
recuperation takes place from the inside of a rail after
the end of the accelerated cooling. As a result, the
pearlitic transformation temperature is pushed up by the
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temperature rise and it becomes impossible to harden
pearlite structures and secure a good wear resistance.
In addition, pearlite blocks coarsen and the ductility of
the rail deteriorates. For those reasons, the present
invention stipulates that accelerated cooling should be
applied until the temperature reaches a temperature not
higher than 550 C.
No lower limit is particularly specified for the
temperature at which accelerated cooling at a railhead
portion is finished but, for securing a good hardness at
a railhead portion and preventing the formation of
martensite structures which are likely to form at
segregated portions and the like in a head inner portion,
400 C is the lower limit temperature, substantially.
Second, explanations are given regarding the
conditions of accelerated cooling at the head, web and
base portions of a rail, that are stipulated in the item
16, for preventing the formation of pro-eutectoid
cementite structures.
In the first place, the range of an accelerated
cooling rate is explained. When an accelerated cooling
rate is below 1 C/sec., with the chemical composition
according to the present invention, it becomes difficult
to prevent the formation of pro-eutectoid cementite
structures. When an accelerated cooling rate exceeds
10 C/sec., on the other hand, with the chemical
composition according to the present invention,
martensite structures form at segregated portions in the
web and base portions of a rail and the toughness of the
rail significantly deteriorates. For those reasons, an
accelerated cooling rate is limited in the range from 1
to 10 C/sec.
Note that the accelerated cooling rate mentioned
above is not a cooling rate during cooling but an average
cooling rate from the beginning to the end of accelerated
cooling. Therefore, as far as an average cooling rate
from the beginning to the end of accelerated cooling is
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within the range specified above, it is possible to
suppress the formation of pro-eutectoid cementite
structures.
Next, explanations are given regarding the
temperature range of accelerated cooling. When
accelerated cooling is finished at a temperature above
650 C, an excessive thermal recuperation takes place from
the inside of a rail after the end of the accelerated
cooling. As a result, pearlite structures are prevented
from forming by the temperature rise and, instead, pro-
euteotoid cementite structures form. For these reasons,
the present invention stipulates that accelerated cooling
should be applied until the temperature reaches a
temperature not higher than 650 C.
No lower limit is practically specified for the
temperature at which accelerated cooling is finished but,
for suppressing the formation of pro-eutectoid cementite
structures and preventing the formation of martensite
structures at the segregated portions in a web portion,
500 C is the lower limit temperature, substantially.
(10) Reasons for limiting the heat treatment conditions
of the web and base portions of a rail
For the purpose of thoroughly preventing the
formation of pro-eutectoid cementite structures in the
web and base toe portions of a rail, a restrictive heat
treatment is applied in addition to the cooling explained
above. Here, the conditions of the heat treatment of the
web and base toe portions of a rail are explained.
First, the conditions of the heat treatment of the
web portion of a rail stipulated in the items 19 and 20
are explained. Explanations begin with the time period
from the end of hot rolling to the beginning of
accelerated cooling at the web portion of a rail. When
the time period from the end of hot rolling to the
beginning of accelerated cooling at the web portion of a
rail exceeds 100 sec., with the chemical composition
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according to the present invention, pro-eutectoid
cementite structures form in the web portion of the rail
before the accelerated cooling and the fatigue strength
and toughness of the rail deteriorate. For those
reasons, the time period till the beginning of
accelerated cooling is limited to not longer than 100
sec.
No lower limit is particularly specified for the
time period from the end of hot rolling to the beginning
of accelerated cooling at the web portion of a rail but,
to make uniform the size of austenite grains in the web
portion of a rail and mitigating the temperature
unevenness occurring during rolling, it is desirable to
begin accelerated cooling after the lapse of not less
than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the range of
the cooling rate of accelerated cooling at the web
portion of a rail. When a cooling rate is below
2 C/sec., with the chemical composition according to the
present invention, it becomes difficult to prevent the
formation of pro-eutectoid cementite structures in the
web portion of a rail. When a cooling rate exceeds
20 C/sec., on the other hand, with the chemical
composition according to the present invention,
martensite structures form at the segregation bands in
the web portion of a rail and the toughness of the web
portion of the rail significantly deteriorates. For
those reasons, an accelerated cooling rate at the web
portion of a rail is limited in the range from 2 to
20 C/sec.
Note that the accelerated cooling rate at the web
portion of a rail mentioned above is not a cooling rate
during cooling but an average cooling rate from the
beginning to the end of accelerated cooling. Therefore,
as long as an average cooling rate from the beginning to
the end of accelerated cooling is within the range
specified above, it is possible to suppress the formation
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of pro-eutectoid cementite structures.
Next, explanations are given regarding the
temperature range of accelerated cooling at the web
portion of a rail. When accelerated cooling is finished
at a temperature above 650 C, an excessive thermal
recuperation takes place from the inside of a rail after
the end of the accelerated cooling. As a result, pro-
eutectoid cementite structures form due to the
temperature rise before pearlite structures form in a
sufficient amount. For those reasons, the present
invention stipulates that accelerated cooling should be
applied until the temperature reaches a temperature not
higher than 650 C.
No lower limit is particularly specified for the
temperature at which accelerated cooling is finished but,
for suppressing the formation of pro-eutectoid cementite
structures and preventing the formation of martensite
structures which form, more at segregated portions, in a
web portion, 500 C is the lower limit temperature
substantially.
Next, the reasons are explained in detail for
limiting the time period from the end of hot rolling to
the beginning of heating at the web portion of a rail and
the temperature range of the heating in their respective
ranges in the items 22 and 23.
First, explanations are given regarding the time
period from the end of hot rolling to the beginning of
heating at the web portion of a rail. When the time
period from the end of hot rolling to the beginning of
heating at the web portion of a rail exceeds 100 sec.,
with the chemical composition according to the present
invention, pro-eutectoid cementite structures form in the
web portion of the rail before the heating, and, even
though the web portion is heated, the pro-eutectoid
cementite structures remain the subsequent heat treatment
and the fatigue strength and toughness of the rail
deteriorate. For those reasons, the time period till the
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beginning of heating is limited to not longer than 100
sec.
No lower limit is particularly specified for the
time period from the end of hot rolling to the beginning
of heating at the web portion of a rail but, for
mitigating the temperature unevenness occurring during
rolling and carrying out the heating accurately, it is
desirable to begin the heating after the lapse of not
less than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the
temperature range of heating at the web portion of a
rail. When the temperature rise of heating is less than
C, pro-eutectoid cementite structures form in the web
portion of a rail before the subsequent accelerated
15 cooling and the fatigue strength and toughness of the web
portion of the rail deteriorate. When the temperature
rise of heating exceeds 100 C, on the other hand,
pearlite structures coarsen after heat treatment and the
toughness of the web portion of a rail deteriorates. For
20 those reasons, the temperature rise of heating at the web
portion of a rail is limited in the range from 20 C to
100 C.
Next, the reasons are explained for specifying the
conditions of the Ileat treatment of the base toe portions
of a rail in the items 18 and 20. First, explanations
are given regarding the time period from the end of hot
rolling to the beginning of accelerated cooling at the
base toe portions of a rail. When the time period from
the end of hot rolling to the beginning of accelerated
cooling at the base toe portions of a rail exceeds 60
sec., with the chemical composition according to the
present invention, pro-eutectoid cementite structures
form in the base toe portions of the rail before the
accelerated cooling and the fatigue strength and
toughness of the rail deteriorate. For those reasons,
the time period till the beginning of accelerated cooling
is limited to not longer than 60 sec.
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No lower limit is particularly limited for the time
period from the end of hot rolling to the beginning of
accelerated cooling at the base toe portions of a rail
but, to make uniform the size of austenite grains in the
base toe portions of a rail and mitigating the
temperature unevenness occurring during rolling, it is
desirable to begin accelerated cooling after the lapse of
not shorter than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the range of
the cooling rate of accelerated cooling at the base toe
portions of a rail. When a cooling rate is below
5 C/sec., with the chemical composition according to the
present invention, it becomes difficult to suppress the
formation of pro-eutectoid cementite structures in the
base toe portions of a rail. When a cooling rate exceeds
C/sec., on the other hand, with the chemical
composition according to the present invention,
martensite structures form in the base toe portions of a
rail and the toughness of the base toe portions of the
20 rail significantly deteriorates. For those reasons, an
accelerated cooling rate at the base toe portions of a
rail is limited in the range from 5 to 20 C/sec.
Note that the accelerated cooling rate at the base
toe portions of a rail mentioned above is not a cooling
rate during cooling but an average cooling rate from the
beginning to the end of accelerated cooling. Therefore,
as far as the average cooling rate from the beginning to
the end of accelerated cooling is within the range
specified above, it is possible to suppress the formation
of pro-eutectoid cementite structures.
Next, explanations are given regarding the
temperature range of accelerated cooling at the base toe
portions of a rail. When accelerated cooling is finished
at a temperature above 650 C, an excessive thermal
recuperation takes place from the inside of a rail after
the end of accelerated cooling. As a result, pro-
eutectoid cementite structures form due to the
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temperature rise before pearlite structures form in a
sufficient amount. For those reasons, the present
invention stipulates that accelerated cooling should be
applied until the temperature-reaches a temperature not
higher than 650 C.
Next, the reasons are explained in detail for
limiting the time period from the end of hot rolling to
the beginning of heating at the base toe portions of a
rail and the temperature range of the heating in their
respective ranges in the items 21 and 23.
First, explanations are given regarding the time
period from the end of hot rolling to the beginning of
heating at the base toe portions of a rail. When the
time period from the end of hot rolling to the beginning
of heating at the base toe portions of a rail exceeds 60
sec., with the chemical composition according to the
present invention, pro-eutectoid cementite structures
form in the base toe portions of the rail before the
heating, and, even though the base toe portions are
heated thereafter, the pro-eutectoid cementite structures
remain the subsequent heat treatment and the fatigue
strength and toughness of the rail deteriorate. For
those reasons, the time period till the beginning of
heating is limited to not longer than 60 sec.
No lower limit is particularly limited for the time
period from the end of hot rolling to the beginning of
heating at the base toe portions of a rail but, for
mitigating the temperature unevenness occurring during
rolling and carrying out the heating accurately, it is
desirable to begin the heating after the lapse of not
less than 5 sec. from the end of hot rolling.
Next, explanations are given regarding the
temperature range of heating at the base toe portions of
a rail. When the temperature rise of heating is less
than 50 C, pro-eutectoid cementite structures form in the
base toe portions of a rail before the subsequent
accelerated cooling and the fatigue strength and
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toughness of the base toe portions of the rail
deteriorate. When the temperature rise of heating
exceeds 100 C, on the other hand, pearlite structures
coarsen after the heat treatment and the toughness of the
base toe portions of a rail deteriorates. For those
reasons, the temperature rise of heating at the base toe
portions of a rail is limited in the range from 50 C to
100 C.
With regard to the conditions of a railhead portion
in the event of applying the above heat treatment, it is
desirable to set the time period from the end of hot
rolling to the heat treatment at not longer than 200 sec.
and the area reduction ratio at the final pass of the
finish hot rolling at 6% or more, or it is more desirable
to apply continuous finish rolling of two or more passes
with a time period of not longer than 10 sec. between
passes at an area reduction ratio of 1 to 30% per pass.
(11) Reasons for limiting the length of a rail after hot
rolling
Here, the reasons are explained in detail for
limiting the length of a rail after hot rolling in the
items 5 and 27.
When the length of a rail after hot rolling exceeds
200 m, the temperature difference between the ends and
the middle portion and moreover between the two ends of
the rail after the rolling becomes so large that it
becomes difficult to properly control the temperature and
the cooling rate over the whole rail length even though
the above rail production method is employed, and the
material quality of the rail in the longitudinal
direction becomes uneven. When the length of a rail
after hot rolling is less than 100 m, on the other hand,
rolling efficiency lowers and the production cost of the
rail increases. For these reasons, the length of a rail
after hot rolling is limited in the range from 100 to 200
m.
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Note that, in order to obtain a product rail length
in the range from 100 to 200 in, it is desirable to secure
a rolling length of the product rail length plus crop
allowances.
(12) Reasons for limiting rolling conditions at hot
rolling
Here, the reasons are explained in detail for
limiting rolling conditions at hot rolling in the items
11 to 14.
When a temperature at the end of hot rolling exceeds
1,000 C, with the chemical composition according to the
present invention, pearlite structures in a railhead
portion are not made fine and ductility is not improved
sufficiently. When a temperature at the end of hot
rolling is below 850 C, on the other hand, it becomes
difficult to control the shape of a rail and, as a
result, to produce a rail satisfying a required product
shape. In addition, pro-eutectoid cementite structures
form immediately after the rolling owing to the low
temperature and the fatigue strength and toughness of a
rail deteriorate. For those reasons, a temperature at
the end of hot rolling is limited in the range from 850 C
to 1,000 C.
when an area reduction ratio at the final pass of
hot rolling is below 6%, it becomes impossible to make a
austenite grain size fine after the rolling of a rail
and, as a consequence, a pearlite block size increases
and it is impossible to secure a high ductility at the
railhead portion. For those reasons, an area reduction
ratio at the final rolling pass is defined as 6% or more.
In addition to the above control of a rolling
temperature and an area reduction ratio, for the purpose
of improving ductility at a railhead portion, 2 or more
consecutive rolling passes are applied at final rolling
and, moreover, an area reduction ratio per pass and a
time period between the passes at final rolling are
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controlled.
Next, the reasons are explained in detail for
limiting an area reduction ratio per pass and a time
period between the passes at final rolling in the item
14.
When an area reduction ratio per pass at final
rolling is less than 1%, austenite grains are not made
fine at all, a pearlite block size is not reduced as a
consequence, and thus ductility at a railhead portion is
not improved. For those reasons, an area reduction ratio
per pass at final rolling is limited to 1% or more. When
an area reduction ratio per pass at final rolling exceeds
30%, on the other hand, it becomes impossible to control
the shape of a rail and thus it becomes difficult to
produce a rail satisfying a required product shape. For
those reasons, an area reduction ratio per pass at final
rolling is limited in the range from 1 to 30%.
When a time period between passes at final rolling
exceeds 10 sec., austenite grains grow after the rolling,
a pearlite block size is not reduced as a consequence,
and thus ductility at a railhead portion is not improved.
For those reasons, a time period between passes at final
rolling is limited to not longer than 10 sec. No lower
limit is particularly specified for a time period between
passes but, for suppressing grain growth, making
austenite grains fine through continuous
recrystallization, and making a pearlite block size small
as a result, it is desirable to make the time period as
short as possible.
Here, the portions of a rail are explained. Fig. 1
shows the denominations of different portions of a rail.
As shown in Fig. 1: the head portion is the portion that
mainly contacts with wheels (reference numeral 1); the
web portion is the portion that is located lower and has
a sectional thickness thinner than the head portion
(reference numeral 5); the base portion is the portion
that is located lower than the web portion (reference
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numeral 6); and the base toe portions are the portions
that are located at both the ends of the base portion 6
(reference numeral 7). In the present invention, the
base toe portions are defined as the regions 10 to 40 mm
apart from both the tips of a base portion. Therefore,
the hACPtoeportions 7 constitute parts of a base
portion 6. Temperatures and cooling conditions in the
heat treatment of a rail are defined by the relevant
represetative values that are measured in the regions 0
to 3 mm in depth from the surfaces of, as shown in Fig.
1, respectively: the center of the rail width at a head
portion 1; the center of the rail width at a base portion
6; the center of the rail height at a web portion 5; and
points 5 mm apart from the tips of base toe portions 7.
Note that it is desirable to make the cooling rates
at the above four measurement points as equal as possible
in order to make uniform the hardness and the structures
in a rail section.
A temperature at the rolling of a rail is
represented by the temperature measured immediately after
rolling at the point in the center of the rail width on
the surface of the head portion 1 shown in Fig. 1.
The present inventors also examined, in a steel rail
of pearlite structures having a high carbon content, the
relationship between the cooling rate capable of
preventing pro-eutectoid cementite structures from
forming at the head inner portion (critical cooling rate
of pro-eutectoid cementite structure formation) and the
chemical composition of the steel rail.
As a result of heat treatment tests using high-
carbon steel specimens simulating the shape of a railhead
portion, it has been clarified that: there is a
relationship between the chemical composition (C, Si, Mn
and Cr) of a steel rail and the critical cooling rate of
pro-eutectoid cementite structure formation; and C, which
is an element that accelerates the formation of
cementite, has a positive correlation and Si, Mn and Cr,
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which are elements that increase hardenability, have
negative correlations.
On the basis of the above finding, the present
inventors tried to determine, in steel rails containing
over 0.85 mass % C, wherein the formation of pro-
eutectoid cementite structures is conspicuous, the
relationship between the chemical composition (C, Si, Mn
and Cr) of the steel rails and the critical cooling rates
of pro-eutectoid cementite structure formation, by using
multiple correlation analysis. As a result, it has been
found that: the value corresponding to the critical
cooling rate of pro-eutectoid cementite structure
formation at the head inner portion of a steel rail is
obtained by calculating the value of CCR defined by the
equation (4) representing the contribution of chemical
composition (mass %) in the steel rail; and further it is
possible to prevent pro-eutectoid cementite structures
from forming at the railhead inner portion by controlling
the cooling rate at the railhead inner portion (ICR,
c/sec.) to not less than the value of CCR in the heat
treatment of a steel rail:
CCR = 0.6 + 10 x ([%C] - 0.9) - 5 x ([%C] - 0.9) x
[%Si] - 0.17[%Mn] - 0.13[%Cr]
.... (4).
Next, the present inventors studied a method for
controlling a cooling rate at a head inner portion (ICR,
C/sec.) in the heat treatment of a steel rail.
In view of the fact that the entire surface of a
railhead portion is cooled in the event of cooling the
railhead portion in a heat treatment, the present
inventors carried out heat treatment tests using high-
carbon steel specimens simulating the shape of a railhead
portion and tried to find out the relationship between
cooling rates at different positions on the surface of a
railhead portion and a cooling rate at a railhead inner
portion. As a result, it has been confirmed that: a
cooling rate at a railhead inner portion correlates with
a cooling rate at the surface of a railhead top portion
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(TH, C/sec.), the average of cooling rates at the
surfaces of the right and left sides of a railhead
portion (TS, C/sec.) and the average of cooling rates at
the surfaces of the lower chin portions (TJ, C/sec.)
that are located at the boundaries between the head and
web portions on the right and left sides; and the cooling
rate at the railhead inner portion can be evaluated by
using the value of TCR defined by the equation (5)
representing the contribution to the cooling rate at the
railhead inner portion:
TCR = 0.05TH ( C/sec.) + 0.10TS ( C/sec.) + 0.50TJ
( C/sec.)
.... (5).
Note that each of the cooling rates at head side
portions and lower chin portions (TS and TJ, C/sec.) is
the average value of the cooling rates at the respective
positions on the right and left sides of a rail.
Further, the present inventors experimentally
investigated the relationship of the value of TCR with
the formation of pro-eutectoid cementite structures in a
railhead inner portion and structures in the surface
layer of a railhead portion. As a result, it has been
clarified that: the formation of pro-eutectoid cementite
structures in a railhead inner portion correlates with
the value of TCR; and, when the value of TCR is twice or
more the value of CCR calculated from the chemical
composition of a steel rail, pro-eutectoid cementite
structures do not form in the railhead inner portion.
It has further been clarified that, in relation to
the microstructures in the surface layer of a railhead
portion, when the value of TCR is four times or more the
value of CCR calculated from the chemical composition of
a steel rail, the cooling is excessive, bainite and
martensite structures detrimental to wear resistance form
in the surface layer of the railhead portion, and the
service life of the steel rail shortens.
That is, the present inventors have found out that,
in the heat treatment of a railhead portion, it is
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possible to secure an appropriate cooling rate at the
railhead inner portion (ICR, C/sec.), prevent the
formation of pro-eutectoid cementite structures there,
and additionally stabilize pearlite structures in the
surface layer of the railhead portion by controlling the
value of TCR so as to satisfy the expression 4CCR TCR
2CCR.
To sum up, the present inventors have found that, in
a steel rail having a high carbon content: it is possible
to prevent the formation of pro-eutectoid cementite
structures in the head inner portion of the steel rail by
controlling the cooling rate at the head inner portion
(ICR) so as to be not less than the value of CCR
calculated from the chemical composition of the steel
rail; and moreover it is necessary to control the value
of TCR calculated from the cooling rates at the different
positions on the surface of the head portion within the
range regulated by the value of CCR for securing an
appropriate cooling rate at the head inner portion (ICR)
and stabilizing pearlite structures in the surface layer
of the head portion.
Accordingly, the present invention makes it possible
to, in the heat treatment of a high-carbon steel rail
used in a heavy load railway: stabilize pearlite
structures in the surface layer of the head portion; at
the same time, prevent the formation of pro-eutectoid
cementite structures, which are likely to form at the
head inner portion and serve as the origin of fatigue
damage; and, as a consequence, secure a good wear
resistance and improve resistance to internal fatigue
damage.
(13) Reasons for regulating the heat treatment method for
preventing the formation of pro-eutectoid cementite
structures in a railhead inner portion
1) Reasons for defining the equation for calculating the
value of CCR
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The reasons are explained for defining the equation
for calculating the value of CCR in the item 24 as
described above.
The equation for calculating the value of CCR has
been derived from the procedures of: firstly measuring
the critical cooling rate of pro-eutectoid cementite
structure formation through the tests simulating the heat
treatment of a railhead portion; and then clarifying the
relationship between the critical cooling rate of pro-
eutectoid cementite structure formation and the chemical
composition (C, Si, Mn and Cr) of a steel rail by using
multiple correlation analysis. The resulting correlation
equation (4) is shown below. As stated above, the
equation (4) is an experimental regression equation, and
it is possible to prevent the formation of pro-eutectoid
cementite structures by cooling a railhead inner portion
at a cooling rate not lower than the value calculated
from the equation (4):
CCR = 0.6 + 10 x ([%C] - 0.9) - 5 x ([%C) - 0.9) x
[%Si] - 0.17[%Mn] - 0.13[%Cr] .... (4).
2) Reasons for limiting a position and a temperature
range wherein a cooling rate at a railhead inner portion
is regulated
The reasons are explained for determining a position
where a cooling rate at a railhead inner portion is
regulated to be a position 30 mm in depth from a head top
surface in the item 24.
A cooling rate at a railhead portion tends to
decrease from the surface toward the inside thereof.
Therefore, in order to prevent pro-eutectoid cementite
structures from forming at the regions of the railhead
portion where the cooling rate is lower, it is necessary
to secure an adequate cooling rate at the railhead inner
portion. As a result of experimentally measuring the
cooling rates at different positions in a railhead inner
portion, it has been confirmed that: the cooling rate at
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the position 30 mm in depth from a head top surface is
the lowest; and, when an adequate cooling rate is secured
at this position, pro-eutectoid cementite structures are
prevented from forming at the railhead inner portion.
From the results, the position where a cooling rate at a
railhead inner portion is regulated is determined to be a
position 30 ram in depth from a head top surface.
Next, the reasons are explained for defining a
temperature range in which a cooling rate at a railhead
inner portion is regulated in the item 24.
It has been experimentally confirmed that, in a
steel rail having the chemical composition as specified
above, the temperature at which pro-eutectoid cementite
structures form is in the range from 750 C to 650 C.
Therefore, in order to prevent the formation of pro-
eutectoid cementite structures, it is necessary to
control a cooling rate at a railhead inner portion to at
least a certain value or more in the above temperature
range. For those reasons, a temperature range in which a
cooling rate at the position 30 mm in depth from the head
top surface of a steel rail is regulated is determined to
be from 750 C to 650 C.
3) Reasons for defining the equation for calculating the
value of TCR and limiting the range of the value
The reasons are explained for defining the equation
for calculating the value of TCR in the item 25.
The equation for calculating the value of TCR has
been derived from the procedures of: firstly measuring a
cooling rate at a railhead top portion (TH, C/sec.), a
cooling rate at railhead side portions (TS, C/sec.), a
cooling rate at lower chin portions (TJ, C/sec.), and
moreover a cooling rate at a railhead inner portion (ICR,
C/sec.) through the tests simulating the heat treatment
of a railhead portion; and then formulating the cooling
rates at the respective railhead surface portions
according to their contributions to the cooling rate at
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the railhead inner portion (ICR, C/sec.). The resulting
equation (5) is shown below. As stated above, the
equation (5) is an empirical equation and, as far as a
value calculated from the equation (5) is not less than a
certain value, it is possible to secure an adequate
cooling rate at a railhead inner portion and prevent the
formation of pro-eutectoid cementite structures:
TCR = 0.05TH ( C/sec.) + 0.10TS ( C/sec.) + 0.50TJ
( C/sec.)
.... (5).
Note that each of the cooling rates at head side
portions and lower chin portions (TS and TJ, C/sec.) is
the average value of the cooling rates at the respective
positions on the right and left sides of a rail.
Next, the reasons are explained for regulating the
value of TCR so as to satisfy the expression 4CCR TCR
2CCR in the item 25.
When the value of TCR is smaller than 2CCR, a
cooling rate at a railhead inner portion (ICR, C/sec.)
decreases, pro-eutectoid cementite structures form in the
railhead inner portion, and internal fatigue damage is
likely to occur. In addition, in that case, the hardness
at the surface of a railhead portion deteriorates and a
good wear resistance of a rail cannot be secured. When
the value of TCR exceeds 4CCR, on the other hand, cooling
rates at the surface layer of a railhead portion increase
drastically, bainite and martensite structures
detrimental to wear resistance form in the surface layer
of the railhead portion, and the service life of the
steel rail shortens. For those reasons, the value of TCR
is restricted in the range specified by the expression
4CCR TCR 2CCR.
4) Reasons for limiting positions and a temperature range
wherein cooling rates at the surface of a railhead
portion are regulated
In the first place, the reasons are explained for
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determining positions where cooling rates at the surface
of a railhead portion are regulated to be three kinds of
portions; a head top portion head side portions and
lower chin portions, in the item 25.
A cooling rate at a railhead inner portion is
significantly influenced by cooling conditions at the
surface of a railhead portion. The present inventors
experimentally examined the relationship between a
cooling rate at a railhead inner portion and cooling
rates at the surface of a railhead portion. As a result,
it has been confirmed that: a cooling rate at a railhead
inner portion is in good correlation with cooling rates
at three kinds of surfaces, through which heat at a
railhead portion is removed, of the top, the sides (right
and left) and the lower chins (right and left) of the
railhead portion; and a cooling rate at a rail head inner
portion is adequately controlled by adjusting cooling
rates at the surfaces. From the results, the positions
where cooling rates at the surface of a railhead portion
are regulated are determined to be the top, the sides and
the lower chins of the railhead portion.
Next, the reasons are explained for defining a
temperature range in which cooling rates at the three
kinds of surfaces of a railhead portion are regulated in
the item 25.
It has been experimentally confirmed that, in a
steel rail having the chemical composition as specified
above, the temperature at which pro-eutectoid cementite
structures form is in the range from 750 C to 650 C.
Therefore, in order to prevent the formation of pro-
eutectoid cementite structures, it is necessary to
control a cooling rate at a railhead inner portion to at
least a certain value or more in the above temperature
range. However, as the amount of heat removed at a
railhead inner portion is smaller than that removed at
the surface of a railhead portion at the time of the end
of accelerated cooling, the temperature at the railhead
CA 02749503 2011-08-10
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inner portion is higher than that at the surface of the
railhead portion. Accordingly, in order to secure an
adequate cooling rate at a railhead inner portion in the
temperature range down to 650 C, beyond which pro-
eutectoid cementite structures form, it is necessary to
regulate a temperature at the end of accelerated cooling
to below 650 C at the surface of the railhead portion.
As a result of verifying experimentally the temperature
at the end of accelerated cooling at the surface of a
railhead portion, it has been confirmed that, when a
cooling is continued until a surface temperature reaches
500 C, a temperature at the end of cooling at a railhead
inner portion falls to below 650 C. From those results,
a temperature range in which cooling rates at the three
kinds of surfaces of a railhead portion (the top, the
sides and the lower chins of a railhead portion) are
regulated is determined to be from 750 C to 500 C.
Here, the portions of a rail are explained. Fig. 10
shows the denominations of different positions at a
railhead portion. The head top portion means the whole
upper part of a railhead portion (reference numeral 1),
the head side portions mean the whole left and right side
parts of a railhead portion (reference numeral 2), the
lower chin portions mean the whole parts on the left and
right sides at the boundaries between a head portion and
a web portion (reference numeral 3), and the head inner
portion means the part in the vicinity of the position 30
mm in depth from the surface of the railhead top portion
in the center of the rail width (reference numeral 4).
Accelerated cooling rates and temperature ranges of
accelerated cooling in the heat treatment of a rail are
defined by the relevant representative values that are
measured on the surfaces of, or in the regions up to 5 mm
in depth from the surfaces of, as shown in Fig. 10,
respectively: the center of the rail width at a head top
portion 1; the center of the railhead height at head side
portions 2; and the center of the lower chin portions 3.
CA 02749503 2011-08-10
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As a consequence, by controlling temperatures and
cooling rates at the above portions, it is possible to
stabilize pearlite structures in the surface layer of a
head portion and control a cooling rate at a head inner
portion 4, thus secure a good wear resistance at the
surface of the head portion, prevent the formation of
pro-eutectoid cementite structures at the head inner
portion, and, in addition, enhance resistance to internal
fatigue damage. With regard to accelerated cooling
during the heat treatment of a railhead portion, it is
possible to arbitrarily choose, as required, the
application or otherwise of cooling and accelerated
cooling rates in the case of the application at the five
positions, namely a head top portion, head side portions
(right and left) and lower chin portions (right and
left), so that the value of TCR may satisfy the
expression 4CCR TCR 2CCR.
Note that it is desirable to make cooling rates on
both the right and left sides of head side portions and
lower chin portions equal in order to make hardness and
metallographic structures uniform on both the sides of a
railhead portion.
As explained above, in order to prevent the
formation of pro-eutectoid cementite structures at a head
inner portion and stabilize pearlite structures in the
surface layer of a head portion in a steel rail of
pearlite structures having a high carbon content, it is
necessary to control a cooling rate at the head inner
portion (ICR) so as to be not lower than the value of CCR
that is determined by the chemical composition of the
steel rail and corresponds to the critical cooling rate
under which cementite structures form, and, at the same
time, to control cooling rates at the aforementioned
different positions on the surfaces of the railhead
portion so that the value of TCR may fall within the
specified range.
It is desirable that the metallographic structure of
CA 02749503 2011-08-10
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a steel rail produced through a heat treatment method
according to the present invention is composed of
pearlite structures almost over the entire body. In some
choices of chemical composition and accelerated cooling
conditions, pro-eutectoid ferrite structures, pro-
eutectoid cementite structures and bainite structures may
form in very small amounts in pearlite structures.
However, as long as the amounts of these structures are
very small, their presence in pearlite structures does
not have a significant influence on the fatigue strength
and the toughness of a rail. For this reason, the
structure of the head portion of a steel rail produced
through a heat treatment method according to the present
invention may include pearlite structures in which small
amounts of pro-eutectoid ferrite structures, pro-
eutectoid cementite structures and bainite structures are
mixed.
Examples
(Example 1)
Table 1 shows, regarding each of the steel rails
according to the present invention, chemical composition,
hot rolling and heat treatment conditions, the
microstructure of a head portion at a depth of 5 mm from
the surface thereof, the number and the measurement
position of pearlite blocks having grain sizes in the
range from 1 to 15 wr1, and the hardness of a head portion
at a depth of 5 mm from the surface thereof. Table 1
also shows the amount of wear of the material at a head
portion after 700,000 repetition cycles of Nishihara wear
test are imposed under the condition of forced cooling as
shown in Fig. 4, and the result of tensile test at a head
portion. In Fig. 4, reference numeral 8 indicates a rail
test piece, 9 a counterpart wheel piece, and 10 a cooling
nozzle.
Table 2 shows, regarding each of the comparative
steel rails, chemical composition, hot rolling and heat
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treatment conditions, the microstructure of a head
portion at a depth of 5 mm from the surface thereof, the
number and the measurement position of pearlite blocks
having grain sizes in the range from 1 to 15 m, and the
hardness of a head portion at a depth of 5 mm from the
surface thereof. Table 2 also shows the amount of wear
of the material at a head portion after 700,000
repetition cycles of Nishihara wear test are imposed
under the condition of forced cooling as shown in Fig. 4,
and the result of tensile test at a head portion.
Note that any of the steel rails listed in Tables 1
and 2 was produced under the conditions of a time period
of 180 sec. from hot rolling to heat treatment and an
area reduction ratio of 6% at the final pass of finish
hot rolling.
The rails listed in the tables are as follows:
* Steel rails according to the present invention (12
rails), Symbols 1 to 12
The pearlitic steel rails excellent in wear
resistance and ductility having chemical composition in
the aforementioned ranges, characterized in that the
number of the pearlite blocks having grain sizes in the
range from 1 to 15 m is 200 or more per 0.2 mm2 of
observation field at least in a part of the region down
to a depth of 10 mm from the surface of the corners and
top of a head portion.
* Comparative steel rails (10 rails), Symbols 13 to 22
Symbols 13 to 16 (4 rails): the comparative steel
rails, wherein the amounts of C, Si, Mn in alloying are
outside the respective ranges according to the claims of
the present invention.
Symbols 17 to 22 (6 rails): the comparative steel
rails having the chemical composition in the
aforementioned ranges, wherein the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m is
less than 200 per 0.2 mm2 of observation field at least
CA 02749503 2011-08-10
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in a part of the region down to a depth of 10 mm from the
surface of the corners and top of a head portion.
Here, explanations are given regarding the drawings
attached hereto. Fig. 3 is an illustration showing, in a
section, the denominations of the different positions on
the surface of the head portion of a pearlitic steel rail
excellent in wear resistance and ductility according to
the present invention and the region where wear
resistance is required. Fig. 4 is an illustration
showing an outline of a Nishihara wear tester. In Fig.
4, reference numeral 8 indicates a rail test piece, 9 a
counterpart wheel piece, and 10 a cooling nozzle. Fig. 5
is an illustration showing the position from which a test
piece for the wear test referred to in Tables. 1 and 2 is
cut out. Fig. 6 is an illustration showing the position
from which a test piece for the tensile test referred to
in Tables. 1 and 2 is cut out.
Further, Fig. 7 is a graph showing the relationship
between the carbon contents and the amounts of wear loss
in the wear test results of the steel rails according to
the present invention shown in Table 1 and the
comparative steel rails shown in Table 2, and Fig. 8 is a
graph showing the relationship between the carbon
contents and the total elongation values in the tensile
test results of the steel rails according to the present
invention shown in Table 1 and the comparative steel
rails shown in Table 2.
The tests were carried out under the following
conditions:
* Wear test of a head portion
Test equipment: Nishihara wear tester (see Fig. 4)
Test piece shape: Disc shape (30 mm in outer
diameter, 8 mm in thickness)
Test piece machining position: 2 mm in depth from
the surface of a railhead top portion
(see Fig. 5)
Test load: 686 N (contact surface pressure 640 MPa)
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Slip ratio: 20%
Counterpart wheel piece: Pearlitic steel (Hv 380)
Atmosphere: Air
Cooling: Forced cooling by compressed air (flow
rate: 100 Nl/min.)
Repetition cycle: 700,000 cycles
* Tensile test of a head portion
Test equipment: Compact universal tensile tester
Test piece shape: JIS No. 4 test piece equivalent;
parallel portion length, 25 mm;
parallel portion diameter, 6 mm;
gauge length for measurement of
elongation, 21 mm
Test piece machining position: 5 mm in depth from
the surface of a railhead top
portion (see Fig. 6)
Strain speed: 10 mm/min.
Test temperature: Room temperature (20 C)
As seen in Tables 1 and 2, in the cases of the steel
rails according to the present invention in contrast to
the cases of the comparative steel rails, pro-eutectoid
cementite structures, pro-eutectoid ferrite structures,
martensite structures and so on detrimental to the wear
resistance and ductility of a rail did not form and the
wear resistance and ductility were good as a result of
controlling the addition amounts of C, Si and Mn within
the respective prescribed ranges.
In addition, as seen in Fig. 7, in the cases of the
steel rails according to the present invention in
contrast to the cases of the comparative steel rails, the
wear resistance improved as a result of controlling the
carbon contents within the prescribed range. In
particular, in the cases of the steel rails having carbon
contents over 0.85% (Symbols 5 to 12) according to the
present invention in contrast to the cases of the steel
rails having carbon contents of 0.85% or less (Symbols 1
to 4) according to the present invention, the wear
CA 02749503 2011-08-10
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resistance improved further.
In addition, as seen in Fig. 8, in the cases of the
steel rails according to the present invention in
contrast to the cases of the comparative steel rails, the
ductility of the head portions improved as a result of
controlling the numbers of the pearlite blocks having
grain sizes in the range from 1 to 15 m. Thus, it was
possible to prevent fractures such as breakage of a rail
in cold regions.
Table 1
Classi- Symbol Steel Chemical composition
Hot rolling and heat treatment conditions Micro- Number of Hardness
of Amount Tensile
fication
structure pearlite blocks 1 head of wear test
of railof head result of
of head to 15 vm In grain portion
portion size
(5 mm in portion head
(mass%) (5
mm in (per 0.2 mm2) depth from portion
C Si Mn Cr/Mo/V/Nb/B
depth Measurement head Total
/Co/Cu/Ni/Ti
from headsurface) elongation
I /Mg/Ca/Al/Zr _
surface) position
(Hv 10 kgf)
(g) (%)
Area reduction ratio of final rolling:13%
405
1 1 0.68 0.25 0.80 Ni:0.15 Rolling
end temperature: 940 C Pearlite 5 mm in depth 335 1.35 22.5
Accelerated cooling rate: 5 C/sec
:from head surface
Area reduction ratio of final rolling:10%
231
2 2 0.75 0.15 1.31 Cu:0.15 Rolling
end temperature: 950 C Pearlite 4 mm in depth 358 1.24 18.3
Accelerated cooling rate: 4 C/sec
from head surface _
765
Reheating temperature: 870 C
3 3 0.80 0.30 0.98Pearlite 8 mm in depth
395 1.15 20.5
Accelerated cooling rate: 7 C/sec
from head surface
0
Area reduction ratio of final rolling: 9%
321
4 4 0.85 0.45 1 Mo:0.02.00 Rolling
end temperature: 940 C Pearlite 6 mm in depth 405 1.08 16.0
Co:0.210
Accelerated cooling rate: 4 C/sec
from head surface N.)
.
....3
Area reduction ratio of final rolling:12%
380 0.
0021
5 0.87 0.52 1.15 Mg:0. Rolling end temperature: 930 C Pearlite 3 mm
in depth 415 0.68 15.8 to
Ca:0.001201
Accelerated cooling rate: 5 C/sec
from head surface 0
_
w
Area reduction ratio of final rolling: 9%
212
6 6 0.91 0.25 0.60 V:0.04 Rolling end temperature: 980
C Pearlite 1 mm in depth 385 0.85 14.5 N.)
0
Invented ,Accelerated cooling rate: 5
C/sec from head surface
I
I-,
rail Area reduction ratio of final
rolling: 8% 248 1
7 7 0.94 0.75 0.80 Cr:0.45 Rolling
end temperature: 960 C Pearlite 3 mm in depth 389 0.75 12.9
0
Accelerated cooling rate: 3 C/sec
from head surface Ln 1
i-,
Area reduction ratio of final rolling:11%
285 0
8 8 1.01 0.81 1.05 8:0.0012 Rolling
end temperature: 960 C Pearlite 2 mm in depth 448 0.59 11.9
I
Accelerated cooling rate: 6 C/sec
from head surface
. _
Area reduction ratio of final rolling:10%
265
9 9 1.04 0.41 0.75 Cr:0.21 Rolling
end temperature: 950 C Pearlite 3 mm in depth 422 0.62 10.9
Accelerated cooling rate: 5 C/sec
from head surface
. _
_
Area reduction ratio of final rolling:15%
348
0015
10 1.10 0.45 1.65 Zr:0. Rolling end temperature: 935 C Pearlite 6 mm
in depth 452 0.52 11.0
Nb:0.018
Accelerated cooling rate: 6 C/sec
from head surface,
, .
Area reduction ratio of final rolling:10%
325
0130
11 11 1.20 1.21 0.65 Ti:0. Rolling end temperature: 920
C Pearlite 7 mm in depth 478 0.36 10.0
A1:0.0400
.
Accelerated cooling rate: 8 C/sec_
from head surface
_
574
12 12 1.38 1.89 0.20 A2:0.18
Reheating temperature: 900 C
Pearlite 9 mm in depth 415 0.30 11.5
Accelerated cooling rate: 10 C/sec
from head surface
Note: Balance of ch,=mical composition is Fe and unavoidable impurities.
Table 2
Classi- Symbol Steel Chemical composition Hot rolling and heat Micro-
Number of
Hardness of Amount of Tensile test
fication treatment conditions
structure pearlite blocks 1 head wear of result of head
of rail (mass%) of head
to 15 !im in grain portion head portion
C Si Mn Cr/Mo/V/Nb/B portion
size (5 mm in portion Total elongation
/Co/Cu/Ni/Ti (5 mm in (per 0.2 ram') depth from
/Mg/Ca/A1/Zr depth Measurement head
from headposition
surface)
surface)
(Hv 10 kgf) (g) (%)
---
Area reduction ratio of final Pearlite
Low carbon
380
rolling: 13% +
pro- content,
13 13 0.60 0.25 0.80 Ni:0.12
Rolling end temperature: 940 C
eutectoid large wear
from head surface
Accelerated cooling rate:3 C/sec ferrite
1.72
Area reduction ratio of final Pearlite
Pro-eutectoid
205
rolling: 9% +
pro- cementite formed
14 14 1.45 1.75 0.20 A1:0.183 mm in depth
375 0.34
Rolling end temperature: 970 C
eutectoid -. low ductility
from head surface
Accelerated cooling rate:5 C/sec cementite
8.9
Excessive Si,
0
Area reduction ratio of final
370
structure
Mg:0.0015 rolling: 12%
0
15 15 0.87 2 15 1.16 Pearlite
3 mm in depth 435 0.90 embrittled, low t\.)
Ca:0.0012 Rolling end temperature: 930 C
from head surface
ductility ....3
Accelerated cooling rate:5 C/sec
12.0 0.
t.0
Area reduction ratio of final Martensi (Xte Martensite 0
240
rolling: 10%
formed
formed, low
w
16 16 0.75 0.16 2.25 Cu:0.16Pearlite 4 mm in depth
528
Rolling end temperature: 950 C
_-- large wear ductility N.)
from head surface
0
Accelerated cooling rate:4 C/sec
2.45 5.2
Area reduction ratio of final
Fine pearlite
I
I
155
rolling: 5%
--- blocks decreased 0
17 17 1.04 0.41 0.76 Cr:0.21Pearlite 3 mm in depth
432 0.60 m
Rolling end temperature: 960 C
-+ low ductility ----.) 1
Compare-
from head surface 61 I-,
Accelerated cooling rate:5
C/sec ______________________________________ 8.6
tive
0
rail Area reduction ratio of final
Fine pearlite I
102
rolling: 10%
blocks decreased
18 18 1.01 0.81 1.02 B:0.0015 Pearlite
2 mm in depth 452 0.57
Rolling end temperature: 1000 C
-+ low ductility
from head surface
Accelerated cooling rate:5 C/sec
8.8
Area reduction ratio of final
Fine pearlite
rolling: 5%
-- blocks decreased
19 19 0.91 0.26 0.61 V:0.03
Pearlite 1 mm in depth 394 0.82
Rolling end temperature: 990 C
+ low ductility
from head surface
Accelerated cooling rate:5 C/sec
10.0
Area reduction ratio of final
Fine pearlite
56
--
20 20 0.94 0.71 0.75 Cr:0.44
rolling: 59 Pearlite 3 mm in depth 405 0.71
blocks decreased
Rolling end temperature: 1020 C -+ low ductility
from head surface
Accelerated cooling rate:3 C/sec 9.2
Area reduction ratio of final
Fine pearlite
175
21 21 1.20 1.15 0.60
Ti:0.0125 rolling:
5% Pearlite 7 mm in depth 480 0.34 --- blocks decreased
A1:0.0300 Rolling end temperature: 920 C
-+ low ductility
from head surface
Accelerated cooling rate:8 C/sec 7.8
Fine pearlite
56
22 22 1.38 1.75 0.25 A2:0.15 Reheating
temperature: 1050 C --
Pearlite 9 mm in depth 425 0.34 blocks decreased
Accelerated cooling rate:6 C/sec
-+ low ductility
from head surface
6.5
Note: Balance of chemical composition is Fe and unavoidable impurities.
CA 02749503 2011-08-10
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(Example 2)
Table 3 shows, regarding each of the steel rails
according to the present invention, chemical composition,
hot rolling and heat treatment conditions, the
microstructure of a head portion at a depth of 5 mm from
the surface thereof, the number and the measurement
position of pearlite blocks having grain sizes in the
range from 1 to 15 m, and the hardness of a head portion
at a depth of 5 mm from the surface thereof. Table 3
also shows the amount of wear of the material at a head
portion after 700,000 repetition cycles of Nishihara wear
test are imposed under the condition of forced cooling as
shown in Fig. 4, and the result of tensile test at a head
portion.
Table 4 shows, regarding each of the comparative
steel rails, chemical composition, hot rolling and heat
treatment conditions, the microstructure of a head
portion at a depth of 5 mm from the surface thereof, the
number and the measurement position of pearlite blocks
having grain sizes in the range from 1 to 15 m, and the
hardness of a head portion at a depth of 5 mm from the
surface thereof. Table 4 also shows the amount of wear
of the material at a head portion after 700,000
repetition cycles of Nishihara wear test are imposed
under the condition of forced cooling as shown in Fig. 4,
and the result of tensile test at a head portion.
Note that any of the steel rails listed in Tables 3
and 4 was produced under the condition of an area
reduction ratio of 6% at the final pass of finish hot
rolling.
The rails listed in the tables are as follows:
* Steel rails according to the present invention (16
rails), Symbols 23 to 38
The pearlitic steel rails excellent in wear
resistance and ductility having chemical composition in
the aforementioned ranges, characterized in that the
CA 02749503 2011-08-10
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number of the pearlite blocks having grain sizes in the
range from 1 to 15 m is 200 or more per 0.2 mm2 of
observation field at least in a part of the region down
to a depth of 10 mm from the surface of the corners and
top of a head portion.
* Comparative steel rails (16 rails), Symbols 39 to 54
Symbols 39 to 42 (4 rails): the comparative steel
rails, wherein the amounts of C, Si, Mn in alloying were
outside the respective ranges according to the claims of
the present invention.
Symbol 43 (1 rail): the comparative steel rail
having the rail length outside the range according to the
claims-of the present invention.
Symbols 44 and 47 (2 rails): the comparative steel
rails, wherein a time period from the end of rolling to
the beginning of accelerated cooling is outside the range
according to the claims of the present invention.
Symbols 45, 46 and 48 (3 rails): the comparative
steel rails, wherein an accelerated cooling rate at a
head portion is outside the range according to the claims
of the present invention.
Symbols 49 to 54 (6 rails): the comparative steel
rails having the chemical composition in the
aforementioned ranges, wherein the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m is
less than 200 per 0.2 mm2 of observation field at least
in a part of the region down to a depth of 10 mm from the
surface of the corners and top of a head portion.
The tests were carried out under the same conditions
as in Example 1.
As seen in Tables 3 and 4, in the cases of the steel
rails according to the present invention in contrast to
the cases of the comparative steel rails, pro-eutectoid
cementite structures, pro-eutectoid ferrite structures,
martensite structures and so on detrimental to the wear
resistance and ductility of a rail did not form and the
CA 02749503 2011-08-10
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wear resistance and ductility were good as a result of
controlling the amounts of C, Si, Mn in alloying, the
rail lengths at the rolling and the time periods from the
end of rolling to the beginning of accelerated cooling
within the respective prescribed ranges.
In addition, as seen in Tables 3 and 4, in the cases
of the steel rails according to the present invention in
contrast to the cases of the comparative steel rails, the
ductility of the railhead portions improved as a result
of controlling the numbers of the pearlite blocks having
grain sizes in the range from 1 to 15 m. Thus, it was
possible to prevent the fractures such as breakage of a
rail in cold regions.
Table 3
Classi- Symbol Steel Chemical composition (mass%, Rail Time
from end Accelerated Micro- Number of Hardness
of Amount of Tensile
fication length of hot cooling
structure pearlite blocks 1 head wear of test
of rail at hot rolling to
conditions of of head to 15 .1.m in grain portion
head result of
rolling beginning of head
portion portion size (5 mm in portion head
accelerated _____________________________________________________________ (5
mm in (per 0.2 mm2) depth from portion
C Si Mn Cr/Mo/V/Nb/B/ cooling Top:Cooling rate
depth Measurement head Total
Co/Cu/Ni/Ti/ Bottom:Cooling
from head position surface) elongation
Mg/Ca/Al/Zr/N (m) (sec)
end temperature surface) (Hv 10 kgf) (g) (%)
-
223
'
23 23 0.65 - 198 198 9 C/sec
_________________________________________________________________________
Pearlite 3 mm in depth 305 1.45 22.5
530 C from head surface
5 C/sec 445
24 24 0.68 0.25 0.80 Ni:0.15 189
_________________________________________ 185 Pearlite 5 mm in depth 335
1.35 23.5
510 C from head surface
4 C/sec 231
25 25 0.75 0.15 1.31 Cu:0.15 165
_________________________________________ 170 Pearlite 4 mm in depth 358
1.24 18.6
545 C from head surface
i
7 C/sec 285
26 26 0.80 0.30 0.98 - 175
____________________________________________ 185 Pearlite 8 mm in depth
395 1.15 14.0
505 C from head surface
Mo:0.02 4 C/sec
351 0
27 27 0.85 0.45 1.00 150 180 Pearlite 6 mm in
depth ___________________ 405 1.08 16.5
2
Co:0.21 Mg:0.0021 5 C/sec
489 C from head surface õ._
_ ,
405
.....3
28 28 0.87 0.52 1.15 178 178 Pearlite 3 mm in
depth ___________________ 415 0.91 16.2 0.
Ca:0.0012 475 C
from head surface to
0-1
_
c
02 6 C/sec
325 w
29 29 0.91 0.25 0.60 V:0. 155
____________________________________________ 158 Pearlite 1 mm in depth
405 0.83 15.0
N:0.0080 515 C
from head surface N.)
o
I-,
5 C/sec 242
30 30 0.91 0.25 0.60 V:0.04 155 156 Pearlite 1 mm in
depth _____________________ 385 0.85 14.8 1
c
500 C 00 m
Invented
from head surface
_
. . _
. CD i
rail 3 C/sec
268 I-,
31 31 0.94 0.75 0.80 Cr:0.45 165
___________________________________________ 156 Pearlite 3 mm in depth
389 0.75 13.0 I c)
520 C from head surface
_
_
12 C/sec 225
32 32 1.01 - - - 165
______________________________________________ 135 Pearlite 2 mm in depth
398 0.65 10.8
450 C from head surface
-
7 C/sec 305
33 33 1.01 0.40 1.05 Cr:0.25 165
___________________________________________ 155 Pearlite 2 mm in depth
448 0.60 11.8
450 C
from head surface ,
10 C/sec 285
34 34 1.04 0.41 0.75 Cr:0.21 150
_________________________________ 115 ' Pearlite 3 mm in depth 432
0.60 12.0
485 C from head surface
Zr:0.0015 6 C/sec 376
35 35 1.10 0.45 1.65 135
115 Pearlite 3 mm in depth 462 0.50 10.5
Nb:0.018 485 C from head surface
Ti:0.0130 12 C/sec 345
36 36 1.20 1.21 0.65 120
58 Pearlite 2 mm in depth 488 0.38 10.2
A1:0.0400 465 C from head surface
_
_ .
18 C/sec 407
37 37 1.38 1.89 0.20 A.1:0.18
110 _____ 25 Pearlite 3 mm in depth 489 0.31 10.2
495 C from head surface
25 C/sec 305
38 38 1.38 0.15 0.20 B:0.012 100
15 Pearlite 3 mm in depth 465 0.35 10.0
485 C from head surface
Note: Balance of chemical composition is Fe and unavoidable impurities.
Table 4
Classi- Symbol Steel Chemical composition
Ra_ . Time from Accelerated Micro- Number of Hardness of Amount of
wear Tensile test
fica- length end of hot cooling structure
of pearlite head of head result of head
tion of at hot rolling to conditions head
portion blocks 1 to portion portion portion
rail rolling beginning of head
(5 mm in 15 um in (5 mm in
(mass%) of portion depth from
grain size depth from
C Si Mn Cr/Mo/V/Nb/ accelerated Top:Cooling head
surface) (per 0.2 mm2) head Total elongation
B/Co/Cu/Ni/ cooling rate
Measurement surface)
Ti/Mg/Ca/A1 Bottom:
position
/Zr/N Cooling end
(m) (sec) temperature
(Hv 10 kgf) (g) (%)
, _______________________________________________________________
250
Lowest carbon
3 C/sec Pearlite +
39 39 0 2
mm in depth content, large
.60 0.25 0.80 Ni:0.12 __________ 150 198
pro-eutectoid 315 22.0
from head
wear
550 C ferrite
1.72
surface
_
.
=205 Pro-eutectoid
5 C/sec Pearlite +
40 40 1.45 1.75 0.20 A1:0.18 ______ 105
100 pro-eutectoid 3 mm in depth 375 0.34
cementite formed
from head
-> low ductility
520 C cementite
8.2
surface
i,
_______________________________________________________________________________
______________ _ _________
Excessive Si,
320
5 C/sec structure
Mg:0.0015 3 mm in depth
41 41 0.87 2.15 1.16 155 160 Pearlite
_________________________ 435 0.90 embrittled, low
Ca:0.0012 from head
480 C ductility 0
surface
9.0
. _
222
Martensite 0
4 C/sec Martensite formed, 1\.)
Pearlite + 4
mm in depth formed, large .....3
42 42 0.75 0.16 2.25 Cu:0.16
165 180 528 low ductility 0.
martensite
from head __- wear to
480 C ---- 5.2 0-1
surface
2.45
. .
0
_
Compa- -
250
Pro-eutectoid w
rative ---225
Pro-eutectoid
(Exce- 10 C/sec Pearlite +
cementite N.)
rail 3
mm in depth martensite formed, 0
43 34 1.04 0.41 0.75 Cr:0.21 ______ ssive
115 pro-eutectoid 402 formed, large
from head
low ductility
I-,
rail 485 C
cementite
surface
wear
7.8
_
I
length) .
1.85 CO0
.
m
Pearlite +
Pro-eutectoid
12 C/sec trace
215
cementite Pro-eutectoid I-,
Ti:0.0130 2 mm in depth
martensite formed, 1 0
44 36 1.20 1.21 0.65 120 265 ___________ pro-
eutectoid 478 formed, large
---
A1:0.0400 from head
low ductility
= 465 C
cementite at
surface
wear
6.9
1.80
rail ends
. .
.
_
Trace pro-
0.5 C/sec Pearlite +
256
eutectoid
Zr:0.0015 trace 3 mm in depth
45 35 1.10 0.45 1.65 110 115
389 0.98 martensite formed,
Nb:0.018 pro-eutectoid from head
485 C cementite surface low ductility
7.2
35 C/sec
286 Martensite
Martensite formed,
46 30 0.91 0.25 0.60 V:0.04 ___________________
155 156 Pearlite + 1 mm in depth
548
formed, large
low ductility
martensite
from head --- wear
500 C 5.0
surface
2.25
Note: Balance of chemical composition is Fe and unavoidable impurities.
Table 5
Classi- Symbol Steel Chemical composition Rai,
Time from Accelerated Micro- Number of Hardness of Amount of wear
Tensile test
fica- length at end of hot cooling structure
of pearlite head of head result of head
tion of hot rolling to conditions head
portion blocks 1 to portion portion portion
rail rolling beginning of head
(5 mm in 15 pm in (5 mm in
(mass%) of portion depth from
grain size depth from
C Si Mn Cr/mo/v/ accelerated Top:Cooling head
surface) (per 0.2 mm2) head Total elongation
Nb/B/Co/ cooling rate
Measurement surface)
Cu/Ni/Ti/ Bottom:
position
Mg/Ca/A1/ Cooling end
1 , Zr/N (m) (sec)
temperature (HY 10 kgf) (g) (%)
r _
9 C/sec
Pearlite block
47 23 0.65 - - 198 300 Pearlite
3 mm 3i1r512depth
302
1.46 coarsened -0 low
from head
ductility
530 C
surface
18.5
- . .
0.5 C/sec
150 280 Pearlite block
3
-,
48 31 0.94 0.75 0.80 Cr:0.45 165 156 Pearlite
mm in depth Softened, 1.25 coarsened low
from head
pearlite ductility
520 C
-
surface
coarsened 10.5
- .
235
Fine pearlite
6 C/sec
V:0.02
1 mm in depth blocks decreased
49 29 0.91 0.25 0.60 ' 155 215
Pearlite 405 0.83
N:0.0080 from head --o low ductility
515 C
surface
13.5
-
Cl
12 C/sec
205 Fine pearlite
50 32 1.01 - - - 165 205 Pearlite
2 mm in depth 398 0.66 blocks decreased coN.)
from head
-o low ductility .....3
Compa- 450 C
_.surface
10.0
.1.
0.
ratiye . .
to
210
Fine pearlite 01
rail 7 C/sec
co
w
51 33 1.01 0.40 1.05 Cr:0.25 165 235 ___________ '
Pearlite 2 mm in depth 448 0.60 blocks decreased
from head
-o low ductility N.)
450 C
surface
10.6 co
234
r Fine pearlite
8 C/sec
1
Zr:0.0015 3 mm in depth
blocks decreased 00 co
52 35 1.10 0.45 1.65 135 225 Pearlite
462 0.51
Nb:0.018 from head -o low ductility Ni T
485 C
_
,surface
9.8
-I- - -
10
12 C/sec
215 Fine pearlite
T1:0.0130 2 mm in depth
blocks decreased
53 36 1.20 1.21 0.65 120 221 Pearlite
480 0.39
A1:0.0400 from head -o low ductility
465 C
surface
9.5
18 C/sec
251 Fine pearlite
54 37 1.38 1.89 0.20 A1:0.18 110
201 Pearlite 3 mm in depth
480
0.34 blocks decreased
from head
--o low ductility
495 C
9.2
isurface
1
Note: Balance of chemical composition is Fe and unavoidable impurities.
CA 02749503 2011-08-10
- 83 -
(Example 3)
The same tests as in Examples 1 and 2 were carried
out using the steel rails of Example 2 shown in Table 3
and changing the time period from the end of rolling to
the beginning of accelerated cooling and the hot rolling
conditions as shown in Table 6.
As is clear from Table 6, total elongation was
further improved in the cases where the time periods from
the end of rolling to the beginning of accelerated
cooling were not longer than 200 sec., 2 or more passes
of the finish hot rolling were applied, and the times
between rolling passes were not longer than 10 sec.
Table 6
Classi-SymbolSteelRail Time from Hot rolling
conditions AcceleratedMicro- Number of Hardness of Amount Tensile
fica- length end of hot
cooling structure pearlite head of wear test
tion of at hot rolling to
conditions of head blocks 1 to portion of head result of
rail rollingbeginning of
head portion 15 .Lia in (5 mm in portion head
of
portion (5 mm in grain size depth from portion
_
acceler- 3 Time 2 Time 1 pass Time
FinalRollingTop:Coolingdepth (per 0.2 mie) head
Total
ated
passes bet- passes bet- to bet- pass end
rate from head Measurement surface) elongation
cooling to ween to ween final ween tempe-
Bottom: surface) position
final passes final passes passes rature
Cooling end
(m) (sec) % sec % sec _ % _ sec
% C temperature (Hy 10 kgf) (g)
(%)
253
9 C/sec
55 23 198 198 - 6 980
Pearlite 3 mm in 305 1.45 24.5
depth from
530 C
head surface
355
6 C/sec
56 29 155 158 a 980
Pearlite 1 mm in
385
0.88 15.1
depth from
515 C
head surface
0
_
385
6 C/sec
c
57 29 155 158 _ 9 870
Pearlite 1 mm in
385
0.88 15.4 N.)
depth from
.....3
515 C
0.
head surface
to
0-1
6 C/sec
380 c
58 29 155 158 - 20 6 2 1 9 980
Pearlite 1 mm in
385
0.88 15.2 w
depth from
N.)
515 C
c
head surface
Inven-
2C/sec 298
I-,
1
= OD 0
ted 59 31 165 156 - e 960
Pearlite 3 mm in 380 0.80 13.3 .A M
rail
520 C depth from I
I-,
head surface
. I 0
285
12 C/sec
60 32 165 135 8 a 8 3 10 960
Pearlite 2 mm in
398
0.65 11.3
depth from
450 C
head surface _
335
7 C/sec
61 33 165 155 - 7 950
Pearlite 2 mm in 448 0.64 12.0
depth from
450 C
head surface
.
_ _
355
7 C/sec
62 33 165 155 - 20 7 2 1 7 950
Pearlite 2 mm in 448 0.64 12.2
depth from
450 C
head surface
385
7 C/Sec
63 33 165 155 10 1 8 1 8 1 7 950
- Pearlite 2 mm in depth from 448 0.64 12.5
450 C
head surface
-
Table 7
Classi- Symbol Steel Rail Time from Hot
rolling conditions AcceleratedMicro- Number of
Hardness of Amount Tensile
fica- length end of hot
cooling structure pearlite head of wear test
tion of at hot rolling to
conditions of head blocks 1 to portion of head result of
rail rollingbeginning of
head portion 15 um in (5 mm in portion head
of
portion (5 mm in grain size depth from portion
acceler- 3 Time 2 Time 1 pass Time Final Rolling
Top :Cooling depth (per 0.2 mm2) head Total
ated passes bet- passes bet- to bet- pass end
rate from head Measurement surface)
elongation
cooling to ween to ween final ween tempe-
Bottom: surface) position
final passes final passes passes rature
Cooling end
(m) (sec) % sec % sec % sec %
C temperature (HY 10 kgf) (g) (%)
398
8 C/sec
64 35 135 115 18 7 3 1 7 920
Pearlite 3 mm in 462 0.50 10.8
485 C
depth from
head surface
, _______________________________________________________
435
8 C/sec
65 35 135 115 8 1 8 1 8 1 7 920
Pearlite 3 mm in 462 0.50 11.5
depth from
485 C
head surface
0
_
385
12 C/sec
0
66 36 120 58 - 10 900
Pearlite 2 mm in 488 0.38 10.8 N.)
..-3
depth from
465 C 0.
-
head surface
to
.
01-
487 0
18 C/sec
w
67 37 110 25 8 0.5 8 0.5 8 0.5 12
930 Pearlite 3 mm in 489 0.31 10.6
Inven-depth from
N.)
495 C
0
tedhead surface
r L
I 1-,
rail
13.1 1
245
6 C/sec
(Small CO
68 29 155 158 - 5 960
Pearlite 1 mm in
385
0_88 area m
Ln 1
depth from
I-,
515 C
head surface reduction
I c
ratio)
_
. ..
265
11.0
7 C/sec
69 33 165 155 = 20 15 2 15 7 950
Pearlite 2 mm in 448 0.64 (Long time
depth from
between
450 C
head surface,
passes) _
10.5
(Small
7 C/sec
235
area
70 33 165 155 10 2 8 3 8 20 5 950
Pearlite 2 mm in 448 0.64 reduction
depth from
ratio)
head surface
(Long time
450 C
between
passes)
_
CA 02749503 2011-08-10
- 86 -
(Example 4)
Table 8 shows, regarding each of the steel rails
according to the present invention, chemical composition,
the value of CE calculated from the equation (1) composed
of the chemical composition, the production conditions of
a casting before rolling, the cooling method at the heat
treatment of a rail, and the microstructure and the state
of pro-eutectoid cementite structure formation at a web
portion.
Tables 9 and 10 shows, regarding each of the
comparative steel rails, chemical composition, the value
of CE calculated from the equation (1) composed of the
chemical composition, the production conditions of a
casting before rolling, the cooling method at the heat
treatment of a rail, and the microstructure and the state
of pro-eutectoid cementite structure formation at a web
portion.
Note that each of the steel rails listed in Tables
8, 9 and 10 was produced under the conditions of a time
period of 180 sec. from hot rolling to heat treatment at
the railhead portion and an area reduction ratio of 6% at
the final pass of finish hot rolling.
In each of those rails, the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m at
a portion 5 mm in depth from the head top portion was in
the range from 200 to 500 per 0.2 mm2 of observation
field.
The rails listed in the tables are as follows:
* Steel rails according to the present invention (12
rails), Symbols 71 to 82
The rails having the chemical composition in the
aforementioned ranges, wherein the amount of formed pro-
eutectoid cementite structures is reduced at the web
portion of a rail, characterized in that the number of
pro-eutectoid cementite network (NC) at a web portion
does not exceed the value of CE calculated from the
contents of the aforementioned chemical composition.
CA 02749503 2011-08-10
- 87 -
* Comparative steel rails (11 rails), Symbols 83 to 93
Symbols 83 to 88 (6 rails): the comparative steel
rails, wherein the amounts of C, Si, Mn, P, S and Cr in
alloying are outside the respective ranges according to
the claims of the present invention.
Symbols 89 to 93 (5 rails): the comparative steel
rails having the chemical composition in the
aforementioned ranges, wherein the number of pro-
eutectoid cementite network (NC) at a web portion exceeds
the value of CE calculated from the contents of the
aforementioned chemical composition.
Here, explanations are given regarding the drawings
attached hereto. Reference numeral 5 (the region shaded
with oblique lines) in Fig. 1 indicates the region in
which pro-eutectoid cementite structures form along
segregation bands. Fig. 2 is a schematic representation
showing the method of evaluating the formation of pro-
eutectoid cementite network.
As seen in Tables 8, 9 and 10, in the cases of the
steel rails according to the present invention in
contrast to the cases of the comparative steel rails, the
number of the pro-eutectoid cementite network (the number
of intersecting cementite network, NC) forming at a web
portion was reduced to the value of CE or less as a
result of controlling the addition amounts of C, Si, Mn,
P, S and Cr within the respective prescribed ranges.
In addition, the number of the pro-eutectoid
cementite network (the number of intersecting cementite
network, NC) forming at a web portion was reduced to the
value of CE or less also as a result of optimizing the
soft reduction during casting and applying cooling to the
web portion.
As stated above, the number of the pro-eutectoid
cementite network (the number of intersecting cementite
network, NC) forming at a web portion was reduced to the
value of CE or less as a result of controlling the
addition amounts of C, Si, Mn, P, S and Cr within the
CA 02749503 2011-08-10
- 88 -
respective prescribed ranges and, in addition, optimizing
the soft reduction during casting and applying cooling to
the web portion. Thus it was possible to prevent the
deterioration of toughness at the web portion of a rail.
Table 8
Classi- Symbol Chemical composition (mass%) CE *1 Casting
conditions and Microstructure of Formation of pro-eutectoid
fication cooling
method at rail web portion *2 cementite structure in web
of rail heat
treatment portion *3
_ ________________________________________________________
C Si Mn P S Cr Mo/V/Nb/B/Co/Cu/Ni
Number of pro-eutectoid
, /Ti/Mg/Ca/Al/Zr/N cementite network (NC)
Optimization of light Pearlite + trace
71 0.86 0.25 1.02 0.015 0.010 0.21
N:0.0085 20 thickness reduction pro-eutectoid
16
during casting
cementite
Optimization of light Pearlite + trace
72 0.90 0.15 0.65 0.028 0.015 0.25
27 thickness reduction pro-eutectoid 25
during casting
cementite
Optimization of light Pearlite + trace
73 0.93 0.56 1.75 0.015 0.011 0.10
Ni:0.20 25 thickness reduction pro-eutectoid
20
during casting
cementite
Optimization of light Pearlite + trace
74 0.95 0.80 0.11 0.011 0.010 0.78
26 thickness reduction pro-eutectoid 21
during casting
cementite
Optimization of light Pearlite + trace
0
75 0.98 0.40 0.70 0.018 0.024 0.25
26 thickness reduction pro-eutectoid 22
during casting
cementite 0
Optimization of light
N.)
Pearlite + trace
.....3
Co:0.15 thickness
reduction 0.
76 1.00 1.35 0.45 0.012 0.008 0.15
8 pro-eutectoid 5 to
Mo:0.03 during
casting 01
cementite
0
Cooling of web portion
w
Invented
Pearlite + trace
A1:0.10
N.)
rail 77 1.05 0.50 1.00 0.008 0.010 0.35
29 Cooling of web portion pro-
eutectoid 27 0
Cu:0.25
cementite
I I-,
1
Optimization of light
Pearlite + trace
OD 0
Mg:0.0015 thickness
reduction m
78 1.10 1.25 0.65 0.010 0.015 0.12
15 pro-eutectoid 10
Ca:0.0015 during
casting
cementite
Cooling of web portion
I 0
Pearlite + trace
B:0.0012
79 1.13 0.80 0.95 0.012 0.019 0.06
24 Cooling of web portion pro-eutectoid
18
Ti :0.0120
cementite
Pearlite + trace
Nb:0.011
80 1.15 0.70 0.45 0.012 0.009 0.15
23 Cooling of web portion pro-eutectoid
18
V:0.02
cementite
Optimization of light
Pearlite + trace
Zr:0.0015 thickness reduction
81 1.19 1.80 0.55 0.011 0.012 0.08
13 pro-eutectoid 7
A1:0.05 during casting
cementite
Cooling of web portion
Optimization of light
Pearlite + trace
82 1.35 1 thickness
reduction .51 0.35 0.012 0.012 0.15 26 pro-eutectoid 22
during casting
cementite
Cooling of web portion
Note: Balance of chemical composition is Fe and unavoidable impurities.
*1: CE = 60[mass % C] - 10(mass % Si] + 10[mass % Mn] + 500(mass % P] +
50[mass % S] + 30(mass % Cr] - 54
*2: Portion at the center of web centerline is observed with an optical
microscope.
*3: Portion where pro-eutectoid cementite structures are exposed at the center
of web centerline is observed with an optical microscope, and number
of intersections of pro-eutectoid cementite network with two line segments
each 300 fim in length crossing each other at right angles is counted
under a magnification of 200 (see Fig. 2). Number of intersecting pro-
eutectoid cementite network is defined as the total of the intersections
on the two line segments.
Table 9
Classi- Symbol Chemical composition (mass%) CE *CCasting
conditions and Microstructure of Formation of pro-eutectoid
fication cooling
method at rail web portion *2 cementite structure in web
of rail heat
treatment portion *3
C Si Mn P S Cr Mo/V/Nb/S/Co/Cu/Nr
Number of pro-eutectoid .
,--.-. ------ -
/Ti/Mg/Ca/Al/Zr
cementite network (NC)
,.---. --- 4.
39
Optimization of light
Pearlite + trace
Excessive segregation in
Zr:0.0020 thickness
reduction
83 1.45 1.70 0.45 0.015 0.012 0.08
31 pro-eutectoid web portion,
A1:0.04 during
casting
cementite
Excessive cementite
Cooling of web portion
formation
Optimization of light
Pearlite + trace
thickness reduction
84 1.00 2.51 0.51 0.015 0.015 0.25
Co:0.25 2 pro-eutectoid 2
during casting
cementite
I Cooling of web portion
i
1 i
Optimization of light
Pearlite + trace Excessive segregation in
85 0.93 0.50 2.85 0.015 0.020 0.15
38 thickness reduction pro-eutectoid web portion,
during casting
cementite Excessive cementite 0
formation
,
_______________________________________________________________________________
_ _, ___________
35 0
-
N.)
Optimization of light
Pearlite + trace Excessive segregation in .....3
86 0.90 0.25 0.68 0.035 0.015 0.25
30 thickness reduction pro-eutectoid web
portion, 0.
to
during casting
cementite Excessive cementite 01
0
formation
w
,
_______________________________________________________________________________
_________________
Compare-
35
tive Optimization
of light Pearlite + trace Excessive segregation in
0
I-,
rail 87 0.98 0.42 0.65 0.019 0.032 0.25
26 thickness reduction pro-eutectoid web portion,
1
during casting
cementite Excessive cementite 0
k.0
formation
CD m
,
1
58 I-,
Optimization of light Pearlite + trace
Excessive segregation in I 0
88 0.95 0.75 0.15 0.012 0.015 1.25
41 thickness reduction pro-eutectoid web portion,
during casting
cementite Excessive cementite
_______________________________________________________________________________
__________________ formation
No control of light
thickness reduction
Pearlite + trace
34
89 0.98 0.40 0.70 0.018 0.024 0.25
26 during casting pro-eutectoid Excessive pro-
eutectoid
No cooling of web
cementite
cementite formation
portion at heat
treatment
No control of light
thickness reduction
Pearlite + trace
32
A1:0.10 during
casting
90 1.05 0,50 1.00 0.008 0.010 0.35
29 pro-eutectoid Excessive pro-eutectoid
Cu:0.25 No cooling
of web
cementite
cementite formation
portion at heat
treatment
____ ___________________
Note: Balance of chemical composition is Fe and unavoidable impurities.
*1: CE = 60(mass % C) - 10(mass % Si] + 10[mass % Mn] + 500(mass % P] +
50(mass % S] + 30[mass % Cr] - 54
*2: Portion at the center of web centerline is observed with an optical
microscope.
*3: Portion where pro-eutectoid cementite structures are exposed at the center
of web centerline is observed with an optical microscope, and number
of intersections of pro-eutectoid cementite network with two line segments
each 300 m in length crossing each other at right angles is counted
under a magnification of 200 (see Fig. 2). Number of intersecting pro-
eutectoid cementite network is defined as the total of the intersections
on the two line segments.
Table 10
ClaSsi- Symbol Chemical composition (mass%) CE *1 Casting
Conditions and Microstructure of Formation of pro-eutectoid
fication cooling
method at rail web portion *2 cementite structure in web
of rail heat
treatment portion *3
C Si Mn P S Cr Mo/V/Nb/B/Co/Cu/Ni
Number of pro-eutectoid
/Ti/Mg/Ca/Al/Zr
cementite network (NC)
,
No control of light
thickness reduction
Pearlite + trace
22
0015 during
casting
91 1.10 1.25 0.65 0.010 0.015 0.12
Mg:0. 15 pro-eutectoid Excessive pro-eutectoid
Ca:0.0015 No cooling of
web
cementite
cementite formation
portion at heat
treatment
_
No control of light
thickness reduction
Compara-
Pearlite + trace 28
Nb:0.011 during
casting _._
tive 92 1.15 0.70 0.45 0.012 0.009 0.15
23 pro-eutectoid Excessive pro-eutectoid
V:0.02 No cooling of
web
rail
cementite cementite formation
portion at heat
treatment
_
No control of light
thickness reduction
0
Pearlite + trace
32
93 1.35 1.51 0.35 0.012 0.012 0.15
26 during casting pro-eutectoid Excessive pro-
eutectoid
No cooling of web
0
cementite
cementite formation N.)
portion at heat
.....3
0.
treatment
to
01
0
Note: Balance of chemical composition is Fe and unavoidable impurities.
w
*1: CE = 60[mass % C] - 10[mass % Si] + 10(mass % Mn] + 500[mass % P] +
50[mass % S] + 30[mass % Cr] - 54
N.)
*2: Portion at the center of web centerline is observed with an optical
microscope. I 0
*3: Portion where pro-eutectoid cementite structures are exposed at the center
of web centerline is observed with an optical microscope, and number
I-,
of intersections of pro-eutectoid cementite network with two line segments
each 300 pm in length crossing each other at right angles is counted k.0 I
under a magnification of 200 (see Fig. 2). Number of intersecting pro-
eutectoid cementite network is defined as the total of the intersections i__.,
0
m
on the two line segments.
1
I
I-,
0
CA 02749503 2011-08-10
- 92 -
(Example 5)
Table 11 shows the chemical composition of the steel
rails subjected to the tests below. Note that the
balance of the chemical composition specified in the
table is Fe and unavoidable impurities.
Tables 12 and 13 show, regarding each of the rails
produced by the production method according to the
present invention using the steels listed in Table 11,
the final rolling temperature, the rolling length, the
time period from the end of rolling to the beginning of
accelerated cooling, the conditions of accelerated
cooling at the head, web and base portions of a rail, the
microstructure, the number and the measurement position
of pearlite blocks having grain sizes in the range from 1
to 15 gri, the result of drop weight test, the hardness at
a head portion, and the value of total elongation in the
tensile test of a head portion.
Tables 14 and 15 show, regarding each of the rails
produced by comparative production methods using the
steels listed in Table 11, the final rolling temperature,
the rolling length, the time period from the end of
rolling to the beginning of accelerated cooling, the
conditions of accelerated cooling at the head, web and
base portions of a rail, the microstructure, the number
and the measurement position of pearlite blocks having
grain sizes in the range from 1 to 15 p.m, the result of
drop weight test, the hardness at a head portion, and the
value of total elongation in the tensile test of a head
portion.
The rails listed in the tables are as follows:
* Heat-treated rails according to the present invention
(11 rails), Symbols 94 to 104
The rails produced under the production conditions
in the aforementioned ranges using the steels having the
chemical composition in the aforementioned ranges.
* Comparative heat-treated rails (8 rails), Symbols 105
CA 02749503 2011-08-10
- 93 -
to 112
The rails produced under the production conditions
outside the aforementioned ranges using the steels having
chemical composition in the aforementioned ranges.
Note that each of the steel rails listed in Tables
12 to 15 were produced under the condition of an area
reduction ratio of 6% at the final pass of finish hot
rolling.
The tests were carried out under the following
conditions:
* Drop weight test
Mass of falling weight: 907 kg
Distance between supports: 0.914 m
Dropping height: 10.6 m
Test temperature: Room temperature (20 C)
Test specimen position: HT, tensile stress on
railhead portion; BT, tensile stress
on rail base portion
* Tensile test of a head portion
Test equipment: Compact universal tensile tester
Test piece shape: JIS No. 4 test piece equivalent;
parallel portion length, 25 mm;
parallel portion diameter, 6 mm;
gauge length for measurement of
elongation, 21 mm
Test piece machining position: 5 mm in depth from
the surface of a railhead top
portion in the center of the
width
Strain speed: 10 mm/min.
Test temperature: Room temperature (20 C)
As seen in Tables 12 to 15, in the steel rails
having high carbon contents as listed in Table 11, in the
cases of the steel rails produced by the production
method according to the present invention wherein
accelerated cooling was applied to the head, web and base
portions of a rail within a prescribed time period after
CA 02749503 2011-08-10
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the end of hot rolling, in contrast to the cases of the
steel rails produced by comparative production methods,
it was possible to suppress the formation of pro-
eutectoid cementite structures and thus prevent the
deterioration of fatigue strength and toughness.
In addition, as seen in Tables 12 to 15, it was
possible to secure a good wear resistance at a railhead
portion, the uniformity of the material quality of a rail
in the longitudinal direction, and a good ductility at a
railhead portion as a result of controlling the
accelerated cooling rate at a railhead portion,
optimizing a rolling length, and controlling a final
rolling temperature.
As stated above, in a steel rail a having a high
carbon content, it was made possible: to suppress the
formation of pro-eutectoid cementite structures
detrimental to the occurrence of fatigue cracks and
brittle cracks by applying accelerated cooling to the
head, web and base portions of the rail within a
prescribed time period after the end of hot rolling in an
attempt to suppress the formation of pro-eutectoid
cementite structures in the head, web and base portions
of the rail; and also to secure a good wear resistance at
the railhead portion, the uniformity of the material
quality of the rail in the longitudinal direction, and a
good ductility at the railhead portion by optimally
selecting an accelerated cooling rate at the railhead
portion, a rail length at rolling, and a final rolling
temperature.
CA 02749503 2011-08-10
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Table 11
Steel Chemical composition (mass%)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
Si:0.35
43 0.86 Mn:1.00
Si:0.25
44 0.90 Mn:0.80
Mo:0.02
Si:0.81
45 0.95 Mn:0.42
Cr:0.54
46 1.00
Si:0.55 Cu:0.35
47 1.00 Mn:0.69
Cr: 0.21
Si:0.75 V:0.030
48 1.01 Mn:0.45 N:0.010
Cr: 0.45
Si:1.35 Zr:0.0017
49 1.11 Mn:0.31
Cr:0.34
Si:0.58 A1:0.08
50 1.19 Mn:0.58
Cr: 0.20
Si:0.45 N:0.0080
51 1.35 Mn:0.35
Cr:0.15
Table 12
Symbol Steel Rolling end Rolling Ti h from end Accelerated
cooling Mic_ .- Number of pearlite Drop
weight Hardness Total
temperature length of hot rolling conditions *2 structure
blocks 1 to 15 pm in test *4 of head elongation
of head to beginning Accelerated Accelerated *3
grain size portion in tensile
portion of accelerated cooling rate cooling end
(per 0.2 mm2) HT:Head tension *5 test of
*1 cooling temperature
Measurement position BT:Base tension head
portion *6
( C) (m) (sec) ( C/sec) ( C)
(Hv) (%)
Head
215 (2 mm in depth
200 1.0 640
Pearlite
portion
from head surface)
HT:No fracture
94 43 1000 200 Web portion 200 1.5 645
Pearlite - BT:No fracture 330 14.0
Base
200 1.2 642
Pearlite -
portion .
Head
220 (2 mm in depth
190 1.2 648
Pearlite
portion
from head surface)
HT:No fracture
95 44 980 200 Web portion 190 1.8 645
Pearlite 320 13.0
BT:No fracture
Base
190 1.8 632
Pearlite
portion
Head
235 (2 mm in depth
185 2.0 630
Pearlite
portion
from head surface)
HT:No fracture
96 45 960 150 Web portion 165 2.5 605
Pearlite - 365 12.5
BT:No fracture
Invented Base
165 2.5 600
Pearlite - 0
produc- portion
_
tion Head
255 (2 mm in depth
165 6.0 450
Pearlite 0
method portion
from head surface) N.)
HT:No fracture .....3
97 45 960 125 Web portion 165 3.0 570
Pearlite - 435 13.4 0.
BT:No fracture to
Base
01
165 4.5 560
Pearlite - 0
portion . .
w
Head
215 (2 mm in depth
145 8.0 450
Pearlite N.)
portion
from head surface) 0
HT:No fracture
98 46 950 150 Web portion 145 3.0 560
Pearlite - 405 10.2
BT:No fracture
I
Base
1.-.0 0
148 4.5 530
Pearlite -
portion
CrN M
.
1
Head
226 (2 mm in depth
150 7.5 465
Pearlite I 0
portion
from head surface)
HT:No fracture
99 47 950 150 Web portion 150 3.5 540
Pearlite - 440 10.5
BT:No fracture
Base
150 5.0 530
Pearlite -
portion
*1: Rolling end temperature of head portion is surface temperature immediately
after rolling. *2: Cooling rates of head, web and base portions are
average figures in the region 0 to 3 mm in depth at the positions specified in
description. *3: Microstructures of head, web and base portions are
observed at a depth of 2 mm at the same positions as specified in above
cooling rate measurement. *4: Drop weight test method is specified in
description. *5: Hardness of head portion is measured at the same position of
head portion as specified in above microstructure observation. *6:
Tensile test method is specified in description.
Table 13
Symbol Steel Rolling end Rolling Tiu Zrom end Accelerated cooling
Mic Number of pearlite Drop weight Hardness Total
tempera- length of hot rolling
conditions *2 structure blocks 1 to 15 vm test *4 of head elongation
ture of to beginning Accelerated Accelerated *3
in grain size portion in tensile
head of acceleratedcooling rate cooling end
(per 0.2 mie) HT:Head tension *5 test of
portion *1 cooling temperature
Measurement position BT:Base tension head
portion *6
(CC) (m) (sec) ( C/sec) ( C)
(HY) (%)
Head
350 (2 net in depth
150 7.5 445 Pearlite
portion
from head surface)
HT:No fracture
100 47 920 115 Web portion 150 3.5 540
_______________________________ Pearlite 445 11.8
BT:No fracture
Base
150 5.0 530 Pearlite
portion . . .
Head
230 (2 mm in depth
125 3.0 530 Pearlite
portion
from head surface)
HT:No fracture
101 48 900 150 Web portion 125 3.5 520
Pearlite - 395 10.8
BT:No fracture
Base
125 4.0 520 Pearlite -
portion
Head
380 (2 mm in depth
Invented 75 8.0 425
Pearlite
portion
from head surface)
produc-
_______________________________________________________________________________
__________________ HT:No fracture
102 49 880 100 Web portion 70 4.5 510
Pearlite - 401 10.4
tion BT:No fracture
Base
method 60 4.5 510
Pearlite - 0
ti
poron
...
Head
400 (2 mm in depth
35 13.0 415 Pearlite
0
portion
from head surface)
HT:No fracture .....3
103 50 870 110 Web portion 35 8.0 505
Pearlite - 485 10.3 0.
BT:No fracture to
Base
01
35 9.5 500 Pearlite -
portion
c
-
w
Head
362 (2 mm in depth
10 23.0 452 Pearlite
IN.)
portion
from head surface) c
HT:No fracture
104 51 900 105 Web portion 10 8.0 515
Pearlite - 465 10.0
BT:No fracture µ..0
Base
--..1 1
10 9.5 520 Pearlite -
c
portion
m
I
1
I-
*
*1: Rolling end temperature of head portion is surface temperature immediately
after rolling. *2: Cooling rates of head, web and base portions are c
average figures in the region 0 to 3 mm in depth at the positions specified in
description. *3: Microstructures of head, web and base portions are
observed at a depth of 2 mm at the same positions as specified in above
cooling rate measurement. *4: Drop weight test method is specified in
description. *5: Hardness of head portion is measured at the same position of
head portion as specified in above microstructure observation. *6:
Tensile test method is specified in description.
Table 14
Symbol Steel Rolling end Rolling Tin. from end Accelerated cooling
Mic_.,- Number of Drop weight Hardness Total
temperature length of hot rolling conditions *2
structure pearlite blocks test *4 of head elongation
of head to beginning Accelerated Accelerated *3 1 to 15 um in
portion in tensile
portion of accelerated cooling rate cooling end grain size
HT:Head tension *5 test of
*1 cooling temperature
(per 0.2 mre) BT:Base tension head
Measurement
portion *6
("C) (m) (sec) ('C/sec) (7C) position
(Hy) (%)
235 (2 mm in
Head
190 4.5 648 Pearlite depth from head
portion
surface) HT:No fracture
BT Fractured
105 44 980 200
Martensite 375 14.0
Web portion 190 13.0 645 (Martensite
+pearlite
. formed)
Base
Martensite
190 11.5 632
.portion +
pearlite
_
Pro-
Head eutectoid
185 0.5 630 -
portion cementite
HT:Fractured
+
pearlite (Pro-eutectoid
Pro-
cementite
eutectoid
formed)
106 45 960 150 Web portion 165 0.4
605 _ 315 12.5
cementite
HT:Fractured
+
pearlite (Pro-eutectoid 0
Pro-
cementite
Compara-
0
Base eutectoid
formed) N.)
tive 165 0.5 600
- .....3
portion cementite
0.
produc-
+
pearlite to
tion . .
01
Head
Martensite 0
method 165 18.0 450
- HT:Fractured 6.4 w
portion +
pearlite
(Martensite
(Martensite
107 45 960 125 web portion 165 3.0 570
Pearlite _ 545 N.)
formed)
formed, low
I
0
I-,
Base
165 4.5 560 Pearlite - BT:No fracture ductility)
portion
. _
0
Pro-
00 m
1
Head eutectoid
150 7.5 465 -
portion cementite
HT:Fractured 0
+
pearlite . (Pro-eutectoid
Pro-
cementite 5.5
eutectoid
formed) (Martensite
106 47 830 150 Web portion 150 3.5 540
- 560
--- cementite
BT:Fractured formed, low
+
pearlite (Pro-eutectoid ductility)
Pro-
cementite
Base eutectoid
formed)
150 5.0 530 -
portion cementite
+ pearlite
*1: Rolling end temperature of head portion is surface temperature immediately
after rolling. *2: Cooling rates of head, web and base portions
are average figures in the region 0 to 3 mm in depth at the positions
specified in description. *3: Microstructures of head, web and base
portions are observed at a depth of 2 mm at the same positions as specified in
above cooling rate measurement. *4: Drop weight test method is
specified in description. *5: Hardness of head portion is measured at the same
position of head portion as specified in above microstructure
observation. *6: Tensile test method is specified in description.
Table 15
Symbol Steel Rolling end Rolling Til, from end Accelerated cooling Mic-o-
Number of Drop weight Hardness Total
temperature length of hot rolling conditions *2
structure pearlite blocks test *4 of head elongation
of head*3
portion in tensile
to beginning Accelerated Accelerated
1 to 15 pin in
portion of accelerated cooling rate cooling end
grain size HT:Head tension *5 test of
*1 cooling temperature
(per 0.2 min2)
BT:Base tension
head
Measurement
portion *6
( C) (m) (sec) ( C/sec) ( C)
position (Hv) (%)
,
305 (2 mm in
Head
150 7.5 445 Pearlite depth from head
portion
surface)
Pro-
HT:No fracture
eutectoid-HT:Fractured
Web portion 150 3.5 685
109 47 920 115 ---
cementite (Pro-eutectoid 445 11.8
+ pearlite
cementite
Pro-
formed)
Base eutectoid
150 5.0 700 -
portion ___
cementite
+ pearlite
215 (2 mm in
Head
125 3.0 530 Pearlite depth from head
portion
surface)
250
HT:No fracture 0
--- Web portion 125 3.5 520
Pearlite -
(Exce-
BT Fractured
Trace pro-
0
110 48 900 ssive
(Pro-eutectoid 395 10.8
N.)
eutectoid
rail
cementite ....3
Compare- Base cementite
0.
length) 125 4.0 520
- formed) to
tive portion at rail
U-1
0
produc- ends +
1 w
tion pearlite
N.)
method
120 (2 mm in 0
7.8
Head
75 8.0 425 Pearlite depth from head
portion
(Pearlite
surface) HT:No fracture 1
111 49 1080 100
401 coarsened I 0
Web portion 70 4.5 510 Pearlite
- BT:No fracture m
,
-P. low I
Base
I-,
60 4.5 510 Pearlite -
ductility)
portion
0
. ¨ _
Pro-
Head eutectoid
35013.0 415 -
portion ____ cementite
HT:Fractured
+ pearlite
(Pro-eutectoid
7.8
Pro-cementite
(Cementite
eutectoid
formed)
112 50 860 110 Web portion 350
8.0 505 435 formed
cementite
HT:Fractured
-. low
_.1- pearlite_
(Pro-eutectoid
ductility)
Pro-
cementite
Base eutectoid
formed)
350 9.5 500
portion --- cementite
+ pearlite
*1: Rolling end temperature of head portion is surface temperature immediately
after rolling. *2: Cooling rates of head, web and base portions
are average figures in the region 0 to 3 mm in depth at the positions
specified in description. *3: Microstructures of head, web and base
portions are observed at a depth of 2 mm at the same positions as specified in
above cooling rate measurement. *4: Drop weight test method is
specified in description. *5: Hardness of head portion is measured at the same
position of head portion as specified in above microstructure
observation. *6: Tensile test method is specified in description.
CA 02749503 2011-08-10
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(Example 6)
Table 16 shows the chemical composition of the steel
rails subjected to the tests below. Note that the
balance of the chemical composition specified in the
table is Fe and unavoidable impurities.
Table 17 shows the reheating conditions of the bloom
(slab) (the values of CT and CM, the maximum heating
temperatures of the bloom (slab) (Tmax) and the retention
times during which the bloom (slab) are heated to 1,100 C
or higher (Mmax)) when the rails are produced by the
production method according to the present invention
using the steels listed in Table 11, and the properties
during hot rolling and after the hot rolling (the surface
properties of the rails thus produced during hot rolling
and after the hot rolling, and the structures and the
hardness of the surface layers of the head portions).
The table also shows the wear test results of the rails
produced by the production method according to the
present invention.
Table 18 shows the reheating conditions of the bloom
(slab) (the values of CT and CM, the maximum heating
temperatures of the bloom (slab) (Tmax) and the retention
times during which the bloom (slab) are heated to 1,100 C
or higher (Mmax)) when the rails are produced by
comparative production methods using the steels listed in
Table 16, and the properties during hot rolling and after
the rolling (the surface properties of the rails thus
produced during hot rolling and after the hot rolling,
and the structures and the hardness of the surface layers
of the head portions). The table also shows the wear
test results of the rails produced by comparative
production methods.
Note that each of the steel rails listed in Tables
17 and 18 was produced under the conditions of a time
period of 180 sec. from hot rolling to heat treatment at
the railhead portion and an area reduction ratio of 6% at
the final pass of finish hot rolling.
CA 02749503 2011-08-10
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Here, explanations are given regarding the drawings
attached hereto. Fig. 9 is an illustration showing an
outline of a rolling wear tester for a rail and a wheel.
In Fig. 9, reference numeral 11 indicates a slider
for moving a rail, on which a rail 12 is placed.
Reference numeral 15 indicates a loading apparatus for
controlling the lateral movement and the load on a wheel
13 driven by a motor 14. During the test, the wheel 13
rolls on the rail 12 and moves back and forth in the
longitudinal direction.
The rails listed in the tables are as follows:
* Heat-treated rails according to the present invention
(11 rails), Symbols 113 to 123
The bloom (slab) and rails produced by the
production method in the aforementioned ranges using the
steels having the chemical composition in the
aforementioned ranges.
* Comparative heat-treated rails (8 rails), Symbols 124
to 131
The bloom (slab) and rails produced by the
production methods outside the aforementioned ranges
using the steels having the chemical composition in the
aforementioned ranges.
The tests were carried out under the following
conditions:
* Rolling wear test
Test equipment: Rolling wear tester (see Fig. 9)
Test piece shape
Rail: 136-1b. rail, 2 m in length
Wheel: Type AAR (920 mm in diameter)
Test load (simulating heavy load railways)
Radial load: 147,000 N (15 tons)
Thrust load: 9,800 N (1 ton)
Repetition cycle: 10,000 cycles
Lubrication condition: Dry
As seen in Tables 17 and 18, in the cases of the
rails produced under the reheating conditions in the
CA 02749503 2011-08-10
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aforementioned ranges in contrast to the cases of the
rails produced under comparative reheating conditions:
the cracks and breaks of a bloom (slab) during rolling
were prevented as a result of optimizing the maximum
heating temperature of the bloom (slab) and the time
period during which the bloom (slab) was heated to a
certain temperature or higher in the reheating process
for hot rolling the bloom (slab) having a high carbon
content as listed in Table 16 into rails; and the
deterioration of wear resistance was prevented as a
result of suppressing the decarburization at the outer
surface layer of a rail and preventing the formation of
pro-eutectoid ferrite structures. Thus, it was possible
to produce high-quality rails efficiently.
CA 02749503 2011-08-10
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Table 16
Steel Chemical composition (mass%)
si/mn/cr/mo/v/Nb/B/co/
Cu/Ni/Ti/Mg/Ca/AliZr/N
Si:0.50
52 0.86 Mn:1.05
Si:0.50 Mo:0.02
53 0.90 Mn:1.05
Cr:0.25
Si: 0.25
54 0.90 Mn:0.65
Cr:0.22
Si: 0.41
55 1.00 Mn:0.70
Cr:0.25
56 1.01
Si:0.81 V:0.03
57 1.01 Mn:0.65 N:0.0080
Cr:0.55
Si:0.45 Cu:0.25
58 1.11 Mn:0.51
Cr:0.34
Si:1.35 Zr:0.0015
59 1.21 Mn:0.15 Ca:0.0020
Cr:0.15
Si:0.35 A1:0.07
60 1.38 Mn:0.12
Table 17
Symbol Steel Value of Value of Reheating conditions of bloom
Properties of rail during and after hot rolling
Wear test
CT *1 CM *2 (slab) for rolling into rail
--__result *5
Maximum heating Retention time Surface condition
Structure of head Hardness of Wear amount
temperature of at 1,100 C or during and after
surface layer *3 head surface
bloom (slab) higher hot rolling
layer *4
Tmax Mmax
( C) (min)
(Hy) (mm)
_______________________________________________________________________________
__________________________________ __
. _________________________________________________________
No bloom (slab)
113 52 1362 487 1325 415 breakage or
rail Pearlite 324 1.95
cracking
,
No bloom (slab)
114 53 1337 465 1305 402 breakage or
rail Pearlite 354 1.89
____________________________________________________________ cracking
No bloom (slab)
115 54 1309 443 1280 385 breakage or
rail Pearlite 395 1.65
cracking
I
_______________________________________________________________________________
__________________________ .._ ___
No bloom (slab)
116 55 1280 420 1270 375 breakage or
rail Pearlite 415 1.45 0
_cracking
No bloom (slab)
0
N.)
117 55 1280 420 1250 345 breakage or
rail Pearlite 424 1.38 .....3
0.
cracking
to
Invented No bloom (slab)
01
0
production 118 56 1277 418 1245 365 breakage or
rail Pearlite 385 1.58 w
method cracking
F- ________________________________________________________________________ .
,___ _____
No bloom (slab)
0
119 57 1277 415 1275 395 breakage or
rail Pearlite 451 1.21
1
_ cracking
,A. 0
m
No bloom (slab)
1
120 57 1277 415 1245 325 breakage or
rail Pearlite 465 1.15 I I-,
0
cracking
No bloom (slab)
121 58 1246 393 1240 350 breakage or
rail Pearlite 435 1.20
cracking
No bloom (slab)
122 59 1213 366 1200 315 breakage or
rail Pearlite 485 0.85
cracking
No bloom (slab)
123 60 1154 320 1140 300 breakage or
rail Pearlite 475 0.75
cracking
*1 CT = 1500 - 140((mass % Cl) - 80((mass % CD2
*2 CM = 600 - 120([mass % CD - 60((mass % C1)2 ,
*3 Observation position of structure of head surface layer: 2 mm in depth from
head top surface at rail width center
*4 Measurement position of hardness of head surface layer: 2 mm in depth from
head top surface at rail width center
*5 Wear test method: See Fig. 9 and description. Wear amount: wear depth in
height direction at rail width center after testing
Table 18
Symbol Steel Value of Value of Reheating conditions of bloom
Properties of rail during and after hot rolling
Wear test
CT *1 CM *2
(slab) for rolling into rail result *5
Maximum heating Retention time Surface condition
Structure of head Hardness of Wear amount
temperature of at 1,100 C or during and after
surface layer *3 head surface
bloom (slab) higher hot rolling
layer *4
Tmax Mmax
( C) (min)
(Hv)___ (mm)
No bloom (slab) Pearlite + pro-eutectoid
124 53 1337 465 1305 600 breakage or rail ferrite
324 3.05
= _________________________________________________________________________
cracking __________ (Much decarburization)
125 54 1309 443 __ 1320 385 Rail cracked Pearlite
385 1.75
Pearlite + pro-eutectoid
126 55 1280 420 1300 485 Rail cracked ferrite
365 2.85
(Much decarburization).
Comparative 127 55 1280 420 1355 345
Bloom (slab) Hot rolling of rail not viable
broke
production
method No bloom (slab)
Pearlite + pro-eutectoid
128 57 1277 415 1275 550breakage or rail ferrite
390 2.64
0
cracking
(Much decarburization) _________________
No bloom (slab) Pearlite + pro-eutectoid
c
N.)
129 58 1246 393 1220 500 breakage or rail ferrite
398 2.45 .....3
0.
---
cracking
jMuch decarburizatiola _____________________________ to
130 58 1213 366 1240 320 Rail cracked
Pearlite 475 0.91 01
'
c
Bloom (slab)
w
131 60 1154 320 1250 300 Hot
rolling of rail not viable
broke
N.)
1
c
I-,
*1 CT = 1500 - 140((mass % CH - 80((mass % C])2
F-,
1
*2 CM = 600 - 120((mass % C]) - 60((mass % C1)2
CD c
*3 Observation position of structure of head surface layer: 2 mm in depth from
head top surface at rail width center un m
1
*4 Measurement position of hardness of head surface layer: 2 mm in depth from
head top surface at rail width center
*5 Wear test method: See Fig. 9 and description. Wear amount: wear depth in
height direction at rail width center after testing I c
CA 02749503 2011-08-10
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(Example 7)
Table 19 shows the chemical composition of the steel
rails subjected to the tests below. Note that the
balance of the chemical composition specified in the
table is Fe and unavoidable impurities.
Tables 20 and 21 show, regarding each of the rails
produced by the heat treatment method according to the
present invention using the steels listed in Table 19,
the rolling length, the time period from the end of
rolling to the beginning of the heat treatment of a base
toe portion, the conditions of the accelerated cooling at
the head, web and base portions of a rail, the
microstructure, the result of a drop-weight test, and the
hardness at a head portion.
Tables 22 and 23 show, regarding each of the rails
produced by the comparative heat treatment methods using
the steels listed in Table 19, the rolling length, the
time period from the end of rolling to the beginning of
the heat treatment of a base toe portion, the conditions
of the accelerated cooling at the head, web and base
portions of a rail, the microstructure, the result of a
drop-weight test, and the hardness at a head portion.
The rails listed in the tables are as follows:
* Heat-treated rails according to the present invention
(11 rails), Symbols 132 to 142
The rails produced under the heat treatment
conditions in the aforementioned ranges using the steels
having the chemical composition in the aforementioned
ranges.
* Comparative heat-treated rails (9 rails), Symbols 143
to 151
The rails produced under the heat treatment
conditions outside the aforementioned ranges using the
steels having the chemical composition in the
aforementioned ranges.
Note that each of the steel rails listed in Tables
20 and 21 was produced under the conditions of a time
CA 02749503 2011-08-10
- 107 -
period of 180 sec. from hot rolling to heat treatment at
the railhead portion and an area reduction ratio of 6% at
the final pass of finish hot rolling.
In each of those rails, the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m at
a portion 5 mm in depth from the head top portion was in
the range from 200 to 500 per 0.2 mm2 of observation
field.
The tests were carried out under the following
conditions:
* Drop-weight test
Mass of falling weight: 907 kg
Distance between supports: 0.914 m
Dropping height: 10.6 m
Test temperature: Room temperature (20 C)
Test specimen position: HT, tensile stress on
railhead portion; BT, tensile stress
on rail base portion
As seen in Tables 20 and 21, and 22 and 23, in the
steel rails having high carbon contents as listed in
Table 19, in the cases of the steel rails produced by the
heat treatment method according to the present invention
wherein preliminary heat treatment was applied to the
base toe portion of a rail within the prescribed time
period after the end of hot rolling and thereafter
accelerated cooling was applied to the head, web and base
portions, in contrast to the cases of the rails produced
by the comparative production methods, the formation of
pro-eutectoid cementite structures was suppressed and
thus the deterioration of fatigue strength and toughness
was prevented.
In addition, as shown in Tables 20 and 21, and 22
and 23, it was made possible to secure a good wear
resistance at the railhead portions as a result of
controlling the accelerated cooling rates at the railhead
portions.
As stated above, in the steel rails having high
CA 02749503 2011-08-10
- 108 -
carbon contents, it was made possible: to suppress the
formation of pro-eutectoid cementite structures
detrimental to the occurrence of fatigue cracks and
brittle cracks as a result of applying accelerated
cooling or heating to the base toe portions of a rail
within the prescribed time period after the end of hot
rolling and thereafter applying accelerated cooling to
the head, web and base portions of the rail; and also to
secure a good wear resistance at a railhead portion as a
result of optimizing the accelerated cooling rate at the
railhead portion.
Table 19
Steel Chemical composition (mass%)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
Si:0.50
61 0.86 Mn:0.80
Si:0.35 Mo:0.03
62 0.90 Mn:0.80
Si:0.80
63 0.95 Mn:0.50
Cr: 0.45
64 1.00
Si:0.55
65 1.00 Mn:0.70
Cr:0.25 1
Si:0.80 V:0.020
66 1.01 Mn:0.45 N:0.010
Cr:0.40
Si:1.45 Zr:0.0020
67 1.11 Mn:0.35 V:0.050
Cr:0.41
Si:0.45 A1:0.07
68 1.19 Mn:0.65
Cr: 0.15
Si:0.45 Cu:0.15
69 1.35 Mn:0.45
Table 20
Symbol Steel Rolling Time up to the Preliminary heat Portion
Accelerated cooling Micro- Drop-weight Hardness of
length start of heat treatment conditions
conditions *2 structure test *4 head portion
treatment of and microstructure of Accelerated
Accelerated *3 *5
base toe base toe portion *1
cooling cooling end HT:Head tension
portion rate
temperature BT:Base tension
(m) (sec) ( C/sec)
( C) (Hv)
. _
Accelerated cooling Head
1.2 640 Pearlite
rate:5 C/sec. portion
HT:No fracture
132 61 198 58 Accelerated cooling end Web portion
1.5 642 Pearlite 329
BT:No fracture
temperature: 645 C Base
1.6 635 Pearlite
Microstructure:pearlite portion
, I
Accelerated cooling Head
1.4 645 Pearlite
rate:6 C/sec. portion
HT:No fracture
133 62 180 52 Accelerated cooling end ;Web portion
1.8 640 Pearlite 329
BT:No fracture
temperature: 635 C Base
1.8 630 Pearlite
Microstructure:pearlite portion
_
Accelerated cooling Head
2.4 625 Pearlite
rate:7 C/sec. portion ,
HT:No fracture 0
P
134 63 185 48 Accelerated cooling end Web portion
2.6 615 Pearlite 385
I
BT:No fracture 0
temperature:625 C Base
Invented 2.0
615 Pearlite rs.)
heat
Microstructure:pearlite,portion
....3
I , I
- IA
q,
treatment Head 6.5
450 Pearlite ul
methodportion
0
Heating by 56 C
HT:No fracture
135 63 158 45 Web portion 3.5
580 Pearlite 455 w
Microstructure:pearlite BT:No fracture
Base
4.0 550 Pearlite 0
portion
Accelerated cooling Head
CD 1
6.0 485 Pearlite
rate:10 C/sec. portionk.r)
0
.
HT:No fracture m
136 64 168 40 Accelerated cooling end Web portion
3.0 530 Pearlite 420 i
BT:No fracture I H
temperature:615 C Base
0
5.5 535 Pearlite
Microstructure:pearlite portion
, .
Head
3.0 485 Pearlite
portion
Heating by 78 C
HT:No fracture
137 65 178 40350
microstructure:pearlite Web portion 3.0
530 Pearlite BT:No fracture
Base
5.5 535 Pearlite
portion
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm
in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the
region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a
depth of 2 mm at the same positions as specified in above cooling
rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as
specified in above microstructure observation.
Table 21
Symbol Steel Rolling Time up to the Preliminary heat Portion
Accelerated cooling Micro- Drop-weight Hardness of
length start of heat treatment conditions
conditions *2 structure test *4 head portion
treatment of and microstructure of Accelerated
Accelerated *3 *5
base toe base toe portion *1
cooling cooling end HT:Head tension
portion rate
temperature BT:Hase tension
(m) (sec) ( C/sec)
( C) (Hv)
___
Head
7.0
440 Pearlite
portion
Heating by 85 C
HT:No fracture
138 65 160 40 Web portion 3.5
545 Pearlite 435
Microstructure:pearlite BT:No fracture
Base
5.5
525 Pearlite
.ortion
Accelerated cooling Head
3.5
530 Pearlite
rate:12 C/sec. portion
HT:No fracture
139 66 155 35 Accelerated cooling end Web portion
3.5 520 Pearlite 385
BT:No fracture .
temperature:545 C Base
4.5
520 Pearlite
Microstructure:pearlite portion
-- __ ¨
Head
Invented 8.5
445 Pearlite 0
ortion
heat Heating by 95 C
HT:No fracture
140 67 145 25 Web portion 4.0
530 Pearlite 425
treatment Microstructure:pearlite
BT:No fracture 0
Base
N.)
method 4.0
525 Pearlite ....3
If
0.
to
Accelerated cooling Head
12.0 425 Pearlite 01
rate:17 C/sec. portion
0
HT:No fracture w
141 68 125 10 Accelerated cooling end Web portion
7.0 515 Pearlite 475
BT:No fracture N.)
temperature:545 C Base
I
9.0
505 Pearlite 0
Microstructure:pearlite portion
I--,
I-,
I
Accelerated cooling Head
20.0 430 Pearlite 0
rate:20 C/sec. portion
CD m
HT:No fracture 1
142 69 105 10 Accelerated cooling end Web portion
7.0 505 Pearlite 495
BT:No fracture
temperature:525 C Base
0
9.0
510 Pearlite
Microstructure:pearlite portion
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm
in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the
region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a
depth of 2 mm at the same positions as specified in above cooling
rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as
specified in above microstructure observation.
Table 22
_______________________________________________________________________________
____________________________ ¨
Symbol Steel Rolling Time up to the Preliminary heat Portion
Accelerated cooling Micro- Drop-weight Hardness of
length start of heat treatment conditions
conditions *2 structure test *4 head portion
treatment of and microstructure of
Accelerated Accelerated *3 *5
base toe base toe portion *1 cooling
cooling end HT:Head tension
portion rate
temperature BT:Base tension
(m) (sec) ( C/sec)
( C) (Hv)
_______________________________________________________________________________
________________________________ - --
, Accelerated cooling Head
1.4
645 Pearlite
rate:5 C/sec. portion
HT:No fracture
Accelerated cooling end Web portion 1.8
640 Pearlite PT:Fractured
143 62 180 52 temperature:700 C
(Pro-eutectoid 329
Microstructure:pro- Base
cementite
1.8
630 Pearlite
eutectoid cementite + portion
formed)
pearlite
_______________________________________________________________________________
____________________________
Accelerated cooling Head
2.4
625 Pearlite
rate:25 C/sec. portion
HT:No fracture
Accelerated cooling end web portion 2.6
615 Pearlite PT:Fractured
144 63 185 48
375
temperature: 625 C
(Martensite
Base
Microstructure: 2.0
615 Pearlite formed) 0
portion
tmartensite + pearlite
_
, .
_______________________________________________________________________________
__________
Head
0
6.5
450 Pearlite N.)
portion
HT:No fracture .....3
Heating by 56 C
Ø
Martensite BT:Fractured
145 63 158 45 Microstructure: web portion 12.5
580 445 to
+ pearlite (Martensite
01
martensite + pearlite
_______________________________________________________________________________
__________________________ 0
Compara- Base
Martensite formed) w
13.0 550
tive heat portion+ pearlite
N.)
,
0
treatment Head
Martensite HT:Fractured I
17.0 485
method portion
+ pearlite (Martensite
Heating by 15 C
Web portion 3.0
530 Pearlite formed) 0
1--,
Microstructure:pro-
m
146 65 178 40
PT:Fractured 514
eutectoid cementite +
Base
(Pro-eutectoid
pearlite 5.5
535 Pearlite I 0
portion
cementite
formed)
_
Pro-
Head
eutectoid
0.5
550
portion
cementite HT:Fractured
1- pearlite_ (Pro-eutectoid
Pro-
cementite
Heating by 85 C
eutectoid formed)
147 65 160 40 Web portion 0.5
545 425
Microstructure:pearlite
cementite BT: Fractured
+ pearlite (Pro-eutectoid
Pro-
cementite
Base
eutectoid formed)
0.5
525
portion
cementite
, . . .
+ pearlite ¨
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm
in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the
region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a
depth of 2 mm at the same positions as specified in above cooling
rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as
specified in above microstructure observation.
Table 23
Symbol Steel Rolling Time up to the Preliminary heat Portion
Accelerated cooling Macro- Drop-weight Hardness of
length start of heat treatment conditions
conditions *2 structure test *4 head portion
treatment of and microstructure of
Accelerated Accelerated *3 *5
base toe base toe portion *1 cooling
cooling end HT:Head tension
portion rate
temperature BT:Base tension
(m) (sec) (
C/sec) ( C) (Hv)
Accelerated cooling Head
3.5 530 Pearlite
rate:1 C/sec. portion
HT:No fracture
Accelerated cooling end Web portion
3.5 520 Pearlite BT:Fractured
_
148 66 155 35 temperature:545 C
(Pro-eutectoid 385
Microstructure:pro- Base
cementite
4.5 520 Pearlite
eutectoid cementite + portion
formed)
pearlite
Accelerated cooling Head
6.5 530 Pearlite HT:No fracture
rate:12 C/sec. portion
245
BT:Fractured
Accelerated cooling end Web portion
3.5 520 Pearlite
(Excessive
(Trace pro-
149 66 35 temperature:545 C
425
eutectoid
Compara- rail
Microstructure:pro- Base
0
tive heat length)
5.5 520 Pearlite cementite
eutectoid cementite + portion
formed)
treatment
_pearlite
0
method - _
N.)
Head
.....3
8.5 445 Pearlite HT:No fracture 0.
Heating by 150 C portion
to
BT:Fractured
01
150 67 145 25 Microstructure:coarse
Web portion 4.0 530 Pearlite 425
(Pearlite
0
pearlite Base
w
4.0 525 Pearlite coarsened)
..portion
N.)
.
- I 0
Accelerated cooling Head
I-,
20.0 430 Pearlite HT:No fracture
rate:20 C/sec. portion
I--+ I-,
. BT:Fractured
_______________________________________________________________________________
__________________________ I-, I
Accelerated cooling end Web portion
7.0 505 __ Pearlite 0
151 69 155 80
(Pro-eutectoid 495 NJ m
--
temperature: 525 C
I
Base
cementite
Microstructure:pro-
9.0 510 Pearlite
portion
formed) 0
eutectoid cementite
*1: Cooling rate of base toe portion is average figure in the region 0 to 3 mm
in depth at the position specified in description.
*2: Cooling rates of head, web and base portions are average figures in the
region 0 to 3 mm in depth at the positions specified in description.
*3: Microstructures of base toe, head, web and base portions are observed at a
depth of 2 mm at the same positions as specified in above cooling
rate measurement.
*4: Drop-weight test method is specified in description.
*5: Hardness of head portion is measured at same position of head portion as
specified in above microstructure observation.
CA 02749503 2011-08-10
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(Example 8)
Table 24 shows the chemical composition of the steel
rails subjected to the tests below. Note that the
balance of the chemical composition specified in the
table is Fe and unavoidable impurities. Tables 25 and 26
show, regarding each of the rails produced by the heat
treatment method according to the present invention using
the steels listed in Table 24, the rolling length, the
time period from the end of rolling to the beginning of
the heat treatment of a web portion, the heat treatment
conditions and the microstructure of a web portion, the
accelerated cooling conditions and the microstructures of
the head and base portions of a rail, the number of
intersecting pro-eutectoid cementite network (N) in a web
portion, and the hardness at a head portion.
Tables 27, 28 and 29 show, regarding each of the
rails produced by comparative heat treatment methods
using the steels listed in Table 24, the rolling length,
the time period from the end of rolling to the beginning
of the heat treatment of a web portion, the heat
treatment conditions and the microstructure of a web
portion, the accelerated cooling conditions and the
microstructures of the head and base portions of a rail,
the number of intersecting pro-eutectoid cementite
network (N) in a web portion, and the hardness at a head
portion.
The rails listed in the tables are as follows:
* Heat-treated rails according to the present invention
(11 rails), Symbols 152 to 162
The rails produced under the heat treatment
conditions in the aforementioned ranges using the steels
having the chemical composition in the aforementioned
ranges.
* Comparative heat-treated rails (11 rails), Symbols 163
to 173
The rails produced under the heat treatment
conditions outside the aforementioned ranges using the
CA 02749503 2011-08-10
- 114 -
steels having the chemical composition in the
aforementioned ranges.
Note that each of the steel rails listed in Tables
25 and 26, and 27, 28 and 29 were produced under the
conditions of a time period of 180 sec. from hot rolling
to heat treatment at the railhead portion and an area
reduction ratio of 6% at the final pass of finish hot
rolling.
In each of those rails, the number of the pearlite
blocks having grain sizes in the range from 1 to 15 tm at
a portion 5 mm in depth from the head top portion was in
the range from 200 to 500 per 0.2 mm2 of observation
field.
Here, explanations are given regarding the number of
intersecting pro-eutectoid cementite network (N)
mentioned in this example and the method for exposing
pro-eutectoid cementite structures for the measurement
thereof.
Firstly, the method for exposing pro-eutectoid
cementite structures is explained. First, a cross-
sectional surface of the web portion of a rail is
polished with diamond abrasive. Then, the polished
surface is immersed in a solution of picric acid and
caustic soda and pro-eutectoid cementite structures are
exposed. Some adjustments may be required of the
exposing conditions in accordance with the condition of a
polished surface, but, basically, desirable exposing
conditions are: an immersion solution temperature is
80 C; and an immersion time is approximately 120 min.
Secondly, the method for measuring the number of
intersecting pro-eutectoid cementite network (N) is
explained.
An arbitrary point where pro-eutectoid cementite
structures are exposed on a sectional surface of the web
portion of a rail is observed with an optical microscope.
The number of intersections of pro-eutectoid cementite
CA 02749503 2011-08-10
- 115 -
network with two line segments each 300 km in length
crossing each other at right angles is counted under a
magnification of 200. Fig. 2 schematically shows the
measurement method.
The number of the intersecting pro-eutectoid
cementite network is defined as the total of the
intersections on the two line segments each 300 km in
length crossing each other at right angles. Note that,
in consideration of uneven distribution of pro-eutectoid
cementite structures, it is desirable to carry out the
counting at least at 5 observation fields and use the
average of the counts as the representative figure of the
specimen.
The results are shown in Tables 25 and 26, and 28
and 29. In the high carbon steel rails having the
chemical composition listed in Table 24, in the cases of
the steel rails produced by the heat treatment method
according to the present invention wherein the heat
treatment in the aforementioned ranges was applied to the
web portion of a rail within the prescribed time period
after the end of hot rolling and additionally the
accelerated cooling in the aforementioned ranges was
applied to the head and base portions of the rail, in
contrast to the cases of the rails produced by
comparative heat treatment methods, the numbers of
intersecting pro-eutectoid cementite network (N) were
significantly reduced.
In addition, in the cases of the steel rails
produced by the heat treatment method according to the
present invention wherein the accelerated cooling in the
aforementioned ranges was applied, in contrast to the
rails produced by the comparative heat treatment methods,
it was possible to prevent the formation of martensite
structures and coarse pearlite structures, which caused
the deterioration of the toughness and the fatigue
strength at the web portion of a rail, as a result of
CA 02749503 2011-08-10
- 116 -
adequately controlling the cooling rates during the heat
treatment.
In addition, as shown in Tables 25 and 26, and 28
and 29, a good wear resistance was secured at the
railhead portions, as evidenced by the rails produced by
the heat treatment method according to the present
invention (Symbols 155 and 158 to 162), as a result of
controlling the accelerated cooling rates at the railhead
portions.
As stated above, in the steel rails having high
carbon contents, it was made possible: to suppress the
formation of pro-eutectoid cementite structures, which
acted as the origins of brittle fracture and deteriorated
fatigue strength and toughness, as a result of applying
accelerated cooling or heating to the web portion of a
rail within the prescribed time period after the end of
hot rolling and also applying accelerated cooling to the
head and base portions of the rail and, after heating of
the web portion too; and, further, to secure a good wear
resistance at a railhead portion as a result of
optimizing the accelerated cooling rate at the railhead
portion.
CA 02749503 2011-08-10
- 117 -
Table 24
Steel Chemical composition (mass%)
Si/Mn/Cr/Mo/V/Nb/B/Co/
Cu/Ni/Ti/Mg/Ca/Al/Zr/N
1 Si:0.25
70 0.86 1 Mn:0.80
Si:0.25 Cu:0.25
71 0.90 Mn:0.80
Cr: 0.20
Si:0.80 Mo:0.03
72 0.95 Mn:0.50
Cr: 0.25
73 1.00
Si:0.55
74 1.00 Mn:0.65
Cr:0.25
Si:0.80 V:0.02
75 1.01 Mn:0.45 N:0.0080
Cr: 0.40
Si:1.45 Zr:0.0015
76 1.11 Mn:0.25
Cr:0.35
Si:0.85 A1:0.08
77 1.19 Mn:0.15
Si:0.85
78 1.34 Mn:0.15
Table 25
Symbol Steel Rolling Time up
Heat treatment conditions and Portion Accelerated cooling conditions Formation
of pro- Hardness
length to the microstructure of web portion and
microstructure of head and eutectoid cementite of head
start of *1
base portions *2*3 structure in web portion
heat
portion *4 ________ *5
treatment Accelerated
Accelerated Micro- Number of
of web cooling cooling end structure
intersecting pro-
portion rate
temperature eutectoid cementite
(m) (sec) ( C/sec)
( C)network (N (Hv)
---__
Cooling
Head
Segregated
rate:2.0 C/sec. 1.4
640 Pearlite 1
portion
portion
Accelerated Cooling end
152 70 200 98
. _________________ 305
cooling temperatures 635 c
Base
Surface
Microstructure: 1.3
640 Pearlite 0
portion
layer
_______________________ _ ________________ pearlite
Cooling
Head
Segregated
rate:2.5 C/sec. 1.5
645 Pearlite 2
portion
portion
Accelerated Cooling end
153 71 198 90
315
cooling temperature:645 C
0
Base
Surface
Microstructure: 1.6
640 Pearlite 0
. portion
layer
pearlite
c
N.)
Cooling
.....3
Head
Se gregated 0.
Invented rate:3.8 C/sec. 2.9
632 Pearlite 5 to
portion
portion
heat
Accelerated Cooling endtil
154 72 185 88
. - 332 o
treatment cooling temperature:630 C
w
Base
Surface
method Microstructure: 2.8
625 Pearlite 0 N.)
portion
layer 0
pearlite
Cooling
Head
Se 1
rate:1.5 C/sec. 4.9
475 P gregatedearlite
c
portion
portion
Heating Cooling end
155 72 185
82 405 ___ CO I
25 C temperature:642 C
Base
Surface 0
Microstructure: 4.5
635 Pearlite 1 I
portion
layer
pearlite . .
Cooling
Head
Segregated
rate:3.5 C/sec. 3.2
605 Pearlite 6
portion
portion
Heating Cooling end
156 73 180 80
360
46 C temperature:620 C
Base
Surface
Microstructure: 2.8
620 Pearlite 0
portion
layer
pearlite
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end
temperature of web portion are average figures in the region 0
to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in
the region 0 to 3 mm in depth at the positions specified in
description.
*3: Microstructure of head, web and base portions are observed at a depth of 2
mm at the same positions as specified in above cooling rate
measurement.
*4: See description and Fig. 2 for methods of exposing pro-eutectoid cementite
structures and measuring the number of intersecting pro-
eutectoid cementite network (N). N at segregated portion of web is measured at
width center of rail centerline on cross-sectional surface
of web portion. N at surface layer of web portion is measured at a depth of 2
mm at the same position as specified in above microstructure
observation.
*5: Hardness of head portion is measured at the same position of head portion
as specified in above microstructure observation.
Table 26
_
_
start of *1
base portions *2*3 structure in web portion
heat
portion *4 ______ *5
treatment Accelerated
Accelerated Micro- Number of
of web cooling cooling end structure
intersecting pro-
portion rate
temperature eutectoid cementite
(m) (sec) ( C/sec)
( C) network (N) (Hv)
Cooling
Head
Segregated
rate:2.8 C/sec. 2.8
595 Pearlite 8
portion
portion
157 74 170 75.
____________________________________________________________________________
56 C temperature:615 C
Base
Surface
Microstructure:pe 2.4
610 Pearlite 0
portion
layer
arlite
_
Cooling
Head
Segregated
rate:4.0 C/sec. 7.0
480 Pearlite 6
portion
portion
Heating Cooling end
158 74 170 52
442
74 temperature:585 C
0
Base
Surface
Microstructure:pe 4.5
545 Pearlite 0
portion
layer
_arlite
0
_
N.)
Cooling
.....3
Head
Se gregated
rate:6.5 C/sec. 5.5
530 Pearlite 7 0.
portion
portion ko
Accelerated Cooling end0-1
159 75 160 65
378 0
Base
Surface
Invented Microstructure: 4.6
520 Pearlite 0 N.)
portion
layer 0
heat pearlite
_______________________________________________________ __. ___
treatment Cooling
Head
Segregated I
method rate:9.0 C/sec. 11.0
445 Pearlite 9 I-, o
portion
portion
160 76 145 25
485 UD
98 C temperature:525 C
Base
Surface 0
Microstructure: 6.0
535 Pearlite 1 1
portion
layer
pearlite
Cooling
Head
Segregated
rate:16.0 C/sec. 15.0
425 Pearlite 8
portion
portion
Accelerated Cooling end
161 77 120 18
455
cooling temperature:515 C
Base
Surface
Microstructure: 7.0
505 Pearlite 1
portion
layer
pearlite
Cooling
Head
Segregated
rate:20.0 C/sec. 18.0
435 Pearlite 9
portion
portion
Accelerated Cooling end
162 78 105 10
476
cooling temperature:535 C
Base
Surface
Microstructure: 10.0
521 Pearlite 1
portion
layer
pearlite
_
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end
temperature of web portion are average figures in the region 0
to 3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in
the region 0 to 3 mm in depth at the positions specified in
description.
*3: Microstructure of head, web and base portions are observed at a depth of 2
mm at the same positions as specified in above cooling rate
measurement.
*4: See description and Fig. 2 for methods of exposing pro-eutectoid cementite
structures and measuring the number of intersecting pro-
eutectoid cementite network (N). N at segregated portion of web is measured at
width center of rail centerline on cross-sectional surface
of web portion. N at surface layer of web portion is measured at a depth of 2
mm at the same position as specified in above microstructure
observation.
*5: Hardness of head portion is measured at the same position of head portion
as specified in above microstructure observation.
Table 27
Symbol Steel Rolling Time up Heat treatment conditions and Portion
Accelerated cooling conditions Formation of pro- Hardness
length to the microstructure of web portion and
microstructure of head and eutectoid cementite of head
start of *1
base portions *2*3 structure in web , portion
heat
portion *4 *5
treatment Accelerated
Accelerated Micro- Number of
of web
cooling cooling end structure intersecting pro-
portion rate
temperature eutectoid cementite
(m) (sec) ( C/sec)
( C) network (N) (Hy)
_¨
Cooling
rate:2.0 C/sec. Head
Segregated
1.4 640 Pearlite 21
Cooling end portion
portion
Accelerated temperature: 720 C
______________________________________________________________________________
163 71 198 90
320
cooling Microstructure:
pro-eutectoid Base
Surface
1.5 645 Pearlite 13
cementite + portion
layer
ataELlIt
___________________________________________________________________________
t 4
-- ___________ --
Cooling
rate:24.0 C/sec. Head
2.7 630 Pearlite Segregated
3
Cooling end portion
portion 0
4)
Accelerated
164 72 185 88 temperature:630 C
_________________________________________________ 335
cooling
0
Microstructure:
N
Base
Surface
Compare- martensite + portion 2.5
620 Pearlite
layer
0 .....3
0.
tive heat pearlite
___ lo
in
treatment Cooling
0
method rate:13.0 C/sec. Head
4.7 470 Pearlite Segregated
2
I W
Cooling end portion
portion
i.--
N
Heating
0
I-,
165 72 185 82 temperature:565 C
_________________________________________________ 402 N.)
25 C
Microstructure: Base
Surface 1
martensite + 4.6
630 Pearlite 0 0
portion
layer I m
pearlite
1
_
,
- _ .
I-
.
_
Cooling
Pro- 0
rate:0.5 C/sec. Head
eutectoid Segregated
0.7 590 29
Cooling end portion
cementite + portion
Heating temperature:610 C
_____________________ pearlite __
166 74 170 75 .
334
56 C Microstructure:
Pro-
pro-eutectoid Base
eutectoid Surface
0.6 620 8
I cementite +
portion
cementite + layer
pearlite
pearlite
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end
temperature of web portion are average figures in the region 0 to
3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in
the region 0 to 3 mm in depth at the positions specified in
description.
*3: Microstructure of head, web and base portions are observed at a depth of 2
mm at the same positions as specified in above cooling rate
measurement.
*4: See description and Fig. 2 for methods of exposing pro-eutectoid cementite
structures and measuring the number of intersecting pro-eutectoid
cementite network (N). N at segregated portion of web is measured at width
center of rail centerline on cross-sectional surface of web
portion. N at surface layer of web portion is measured at a depth of 2 mm at
the same position as specified in above microstructure
observation.
*5: Hardness of head portion is measured at the same position of head portion
as specified in above microstructure observation.
Table 28
Symbol Steel Rolling Time up Heat treatment conditions and Portion
Accelerated cooling conditions Formation of pro-
Hardness
length to the microstructure of web portion and
microstructure of head and eutectoid cementite of head
start of *1 base
portions *2*3 structure in web portion
heat
portion *4 *5
treatment
Accelerated Accelerated Micro- Number of
of web
cooling cooling end structure intersecting pro-
portion rate
temperature eutectoid cementite
(in) (sec) ( C/sec)
( C) network (N) (Hv)
-------
---1- -
Cooling
rate:4.2 C/sec. Head
Segregated
7.2 485 Pearlite 35
Cooling end portion
portion
Heating temperature:585 C
167 74 170 52
442
12 C Microstructure:
pro-eutectoid
Base
Surface
cementite + 4.0
550 Pearlite 10
portion
layer
pearlite
. . __
Natural cooling in Head
Segregated
7.2 485 Pearlite 39 0
air portion
portion
Heating Microstructure:
0
168 74 170 -
Pro- 442 N.)
54 C pro-eutectoid
.....3
Base
eutectoid Surface
cementite + Natural
cooling in air 20 0.
portion
cementite + layer to
Compara- pearlite
pearlite
0-1
0
'
tive heat
w
Cooling
treatmentN.)
rate:1.0 C/sec. Head
Segregated
method 5.0
535 Pearlite 34 1 0
Cooling end portion
portion
I-,
Accelerated temperature:550 C
1
378
169 75 160 65
I-,
cooling Microstructure:
IV 0M
pro-eutectoid Base
Surface
4.5 525 Pearlite 11
cementite + portion
layer I o
____________________________________________ pearlite
Cooling
rate:3.5 C/sec. Head
Segregated
5.0 535 Pearlite 25
Cooling end
235 portion
portion
--- temperature:540 C
(Excessive Accelerated
170 75 35 Microstructure:
388
rail cooling
trace pro-
length)
Surface
eutectoid Base
4.5 525 Pearlite 4
cementite at rail Portion
layer
ends + pearlite
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end
temperature of web portion are average figures in the region 0 to
3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in
the region 0 to 3 mm in depth at the positions specified in
description.
*3: Microstructure of head, web and base portions are observed at a depth of 2
mm at the same positions as specified in above cooling rate
measurement.
*4: See description and Fig. 2 for methods of exposing pro-eutectoid cementite
structures and measuring the number of intersecting pro-eutectoid
cementite network (14). N at segregated portion of web is measured at width
center of rail centerline on cross-sectional surface of web portion.
N at surface layer of web portion is measured at a depth of 2 mm at the same
position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion
as specified in above microstructure observation.
Table 29
Symbol steel Rolling Time up Heat treatment conditions and Portion
Accelerated cooling conditions Formation of pro- Hardness
length to the microstructure of web portion and
microstructure of head and eutectoid cementite of head
start of *1 base
portions *2*3 structure in web portion
heat
portion *4 *5
treatment
Accelerated Accelerated Micro- Number of
of web cooling
cooling end structure intersecting pro-
portion rate
temperature eutectoid cementite
(M) (sec) ( C/sec)
( C) network (N) (Hv)
___________ _
Cooling
Head
Segregated
rate:9.0 C/sec. 12.5
445 Pearlite 9
portion
portion
Heating Cooling end
171 76 145 25
485
165 C temperature:525 C
Base
Surface
Microstructure: 5.0
535 Pearlite 1
portion
layer
coarse pearlite
____________________________________________ --
,
Cooling
rate:16.0 C/sec. Head
Segregated
18.0 455 Pearlite 38
Cooling end portion
portion
Compare- Accelerated temperature:515 C
____________________________________________________________ 0
465
172 77 120 125
tive heat --- cooling Microstructure:
treatment pro-eutectoid Base
Surface 0
6.0 505 Pearlite 14 N.)
method cementite + portion
layer .....3
0.
____________________________________________ pearlite
to
Pro-
01
0
Natural cooling in Head
eutectoid Segregated w
Natural cooling in air
40
air portion
cementite + portion N.)
Accelerated Ma
I crostructure: pearlite 0
173 78 105 -
345 '-
cooling pro-eutectoid
Pro-
1
cementite + Base
eutectoid Surface NJ
Natural cooling in air
240
NJ
m
pearlite portion
cementite + layer
1
pearlite
I
I-,
co
*1: Heating temperature, accelerated cooling rate, and accelerated cooling end
temperature of web portion are average figures in the region 0 to
3 mm in depth at the positions specified in description.
*2: Accelerated cooling rates of head and base portions are average figures in
the region 0 to 3 mm in depth at the positions specified in
description.
*3: Microstructure of head, web and base portions are observed at a depth of 2
mm at the same positions as specified in above cooling rate
measurement.
*4: See description and Fig. 2 for methods of exposing pro-eutectoid cementite
structures and measuring the number of intersecting pro-eutectoid
cementite network (N). N at segregated portion of web is measured at width
center of rail centerline on cross-sectional surface of web portion.
N at surface layer of web portion is measured at a depth of 2 mm at the same
position as specified in above microstructure observation.
*5: Hardness of head portion is measured at the same position of head portion
as specified in above microstructure observation.
,
CA 02749503 2011-08-10
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(Example 9)
Table 30 shows the chemical composition of the steel
rails subjected to the tests below. Note that the
balance of the chemical composition specified in the
table is Fe and unavoidable impurities.
Tables 31 and 32 show the values of CCR of the
steels listed in Table 30, and, regarding each of the
rails produced through the heat treatment according to
the present invention using the steels listed in Table
30, the rolling length, the time period up to the
beginning of heat treatment, the heat treatment
conditions (cooling rates and the values of TCR) at the
inside and the surface of a railhead portion, and the
microstructure of a railhead portion.
Tables 33 and 34 show the values of CCR of the
steels listed in Table 30, and, regarding each of the
rails produced through the comparative heat treatment
using the steels listed in Table 30, the rolling length,
the time period up to the beginning of heat treatment,
the heat treatment conditions (cooling rates and the
values of TCR) at the inside and the surface of a
railhead portion, and the microstructure of a railhead
portion.
Here, explanations are given regarding the drawings
attached hereto. Fig. 1 is an illustration showing the
denominations of different portions of a rail.
In Fig. 10, the reference numeral 1 indicates the
head top portion, the reference numeral 2 the head side
portions at the right and left sides of the rail, the
reference numeral 3 the lower chin portions at the right
and left sides of the rail, and the reference numeral 4
the head inner portion, which is located in the vicinity
of the position at a depth of 30 mm from the surface of
the head top portion in the center of the width of the
rail.
The rails listed in the tables are as follows:
* Heat-treated rails according to the present invention
CA 02749503 2011-08-10
- 124 -
(11 rails), Symbols 174 to 184
The rails produced by applying heat treatment to the
railhead portions under the conditions in the
aforementioned ranges using the steels having the
chemical composition in the aforementioned ranges.
* Comparative heat-treated rails (10 rails), Symbols 185
to 194
The rails produced by applying heat treatment to the
railhead portions under the conditions outside the
aforementioned ranges using the steels having the
chemical composition in the aforementioned ranges.
Note that any of the steel rails listed in Tables 31
and 32, and 33 and 34 were produced under the conditions
of a time period of 180 sec. from hot rolling to heat
treatment at the railhead portion and an area reduction
ratio of 6% at the final pass of finish hot rolling.
In each of those rails, the number of the pearlite
blocks having grain sizes in the range from 1 to 15 m at
a portion 5 mm in depth from the head top portion was
within the range from 200 to 500 per 0.2 mm2 of
observation field.
As seen in Tables 31 and 32, and 33 and 34, in the
steel rails having high carbon contents as listed in
Table 30, in the cases of the steel rails produced by the
heat treatment method according to the present invention
wherein the cooling rate at a head inner portion (ICR)
was controlled so as to be not lower than the value of
CCR calculated from the chemical composition of a steel
rail, in contrast to the cases of the rails produced by
the comparative heat treatment methods, the formation of
pro-eutectoid cementite structures at a head inner
portion was prevented and resistance to internal fatigue
damage was improved.
In addition, as seen also in Tables 31 and 32, and
33 and 34, it was made possible to prevent the pro-
eutectoid cementite structures detrimental to the
occurrence of fatigue damage from forming at a head inner
CA 02749503 2011-08-10
- 125 -
portion and, at the same time, to prevent the bainite and
martensite structures detrimental to wear resistance from
forming in the surface layer of a railhead portion as a
result of controlling the value of TCR calculated from
the cooling rates at the different positions on the
surface of the railhead portion within the range defined
by the value of CCR with intent to prevent the formation
of pro-eutectoid cementite structures at a railhead inner
portion, or secure the cooling rate at a head inner
portion (ICR), and stabilize the pearlite structures in
the surface layer of a railhead portion.
As described above, in the steel rails having high
carbon contents, it was made possible to prevent pro-
eutectoid cementite structures detrimental to the
occurrence of fatigue damage from forming at a railhead
inner portion and, at the same time, obtain pearlite
structures highly resistant to wear in the surface layer
of a railhead portion as a result of controlling the
cooling rate at the railhead inner portion (ICR) within
the prescribed range and the cooling rates at the
different positions on the surface of the railhead
portion within the prescribed range.
CA 02749503 2011-08-10
- 126 -
Table 30
1 Steel Chemical composition (mass%)
Si Mn Cr Mo/V/Nb/B/Co/Cu
Ni/Ti/Mg/Ca/Al/Zr
79 0.86 0.25 1.15 0.12
1 1
Mo:0.02
80 0.90 0.25 1.21 0.05
81 0.95 0.51 0.78 0.22
82 1.00 0.42 0.68 0.25
Ti:0.0150
83 1.01 0.75 0.35 0.75 B:0.0008
Zr: 0.0017
84 1.11 0.11 0.31 0.31 Ca:0.0021
V:0.02
85 1.19 1.25 0.15 0.15 A1:0.08
86 1.35 1.05 0.25 0.25
Table 31
Symbol Steel Value 2 CCR 4 CCR Rolling Time up Heat Heat treatment
conditions of head surface Microstructure *5
of CCR length to the treatment
*1 start of conditions
heat of head
treatment inner
of head portion
portion Cooling Cooling rate Cooling rate Cooling rate Value of
rate *2 at head top at head side at lower chin TCR *4
(value of portion *3 portion *3 portion *3
ICR) T S
A
(m) (sec) ( C/sec) ( C/sec) ( c/sec) ( C/sec)
_______________________________________________________________________________
____________________________________ ___
Head top
Pearlite
portion
174 79 0.04 0.08 0.16 198 198 0.21 0.5 0.5 0.1 0.13
Head inner
Pearlite
,___
_______________________________________________________________________________
_____________________ portion
Head top
Pearlite
175 80 0.39 0.78 1.56 185 178 0.41 3.0 3.0 1.0 0.95
_portion
Head inner 0
Pearlite
portion
Head top
0
Pearlite N.)
portion
.....3
176 81 0.81 1.62 3.24 185 165 0.91 4.0 3.0
3.0 2.00 ---- 0.
Invented
Head inner to
Pearlite U-1
heat
,portion 0
treatment
Head top w
Pearlite
methodEortion
I
N.)
177 81 0.81 1.62 3.24 175 150 1.05 6.0 4.0
4.0 2.70 c
Head inner
_
I-,
Pearlite
_______________________________________________________________________________
___________________ portion
NJ
1
Head top
0
=
Pearlite --..1 m
178 82 1.24 2.48 4.96 160 135 1.45 5.0 6.0
5.0 3.35 _E9rtion I
I-,
Head inner I 0
Pearlite
_______________________________________________________________________________
___________________ portion
Head top
Pearlite
portion
179 82 1.24 2.48 4.96 160 120 1.74 5.0 5.0 6.0 3.75
Head inner
Pearlite
portion 1
.
____ _
____
*1 CCR ( C/sec.) = 0.6 + 10 x (05C) - 0.9) - 5 x ([%C] - 0.9) x [%Si] -
0.17[%Mh] - 0.13P6Cr]
*2 Cooling rate ( C/sec.) at head inner portion: cooling rate at a depth of 30
mm from head top surface in temperature range from 750 C to
650 C.
*3 Cooling rates at head surface (head top portion, head side portion and
lower chin portion): cooling rate in the region from surface to
mm in depth in temperature range from 750 C to 500 C. Cooling rates at head
side portion and lower chin portion are average figures of
right and left sides of rail.
*4 TCR = 0.05 x T (cooling rate at head top portion, C/sec.) + 0.10 x S
(cooling rate at head side portion, C/sec.) + 0.50 x J (cooling
rate at lower chin portion, C/sec.)
*5 microstructures are observed at a depth of 2 mm (head top portion) and at a
depth of 30 mm (head inner portion) from head top surface.
Table 32
_______________________________________________________________________________
_________ - ______________
Symbol Steel Value 2 CCR 4 CCR Rolling Time up Heat Heat treatment
conditions of head surface Microstructure *5
of CCR length to the treatment
*1 start of conditions
heat of head
treatment inner
of head portion
portion Cooling Cooling
rate Cooling rate Cooling rate Value of
rate *2 at head top at head side at lower chin TCR *4
(value of portion *3 portion *3 portion *3
ICR) T S
A
(m) (sec) ( C/sec) ( C/sec) ( C/sec) ( C/sec) _____- =-=_. _-
__
Head top
Pearlite
portion
180 83 1.13 2.26 4.52 155 110 1.25 6.0
2.0 5.0 3.00
Head inner
Pearlite
portion
Head top
3,30 portion =
181 83 1.13 2.26 4.52 145 80 1.50 8.0
4.0 5.0
Head inner
Pearlite 0
_______________________________________________________________________________
___________________ portion
Invented ________________________ ,
_____________________________________________________________ Head top
0
Pearlite t\.)
182 84 2.49 4.98 9.97 130 65 3.54 6.0
8.0 12.0 7.10 portion 0.
treatment
Head inner
Pearlite t..0
method
portion 01
0
4.80
Head top
Pearlite I w
N.)
183 85 1.64 3.28 6.56 105 35 2.25 4.0
6.0 8.0 _portion ,
Head inner 0
Pearlite
_______________________________________________________________________________
___________________ portion , ts...)
I
Head top
00 0
Pearlite m
1
184 86 2.66 5.32 10.64 120 15 2.25 12.0 8.0 14.0
8.40 portion
Head inner
Pearlite 0
portion
_______________________________________________________________________________
___________________________ l
,
*1 CCR ( c/sec.) = 0.6 + 10 x (P6C) - 0.9) - 5 x ((%C] - 0.9) x [26Si] -
0.17[96Mn] - 0.13[%Cr]
*2 Cooling rate ( C/sec.) at head inner portion: cooling rate at a depth of 30
mm from head top surface in temperature range from 750 C to
650 C.
*3 Cooling rates at head surface (head top portion, head side portion and
lower chin portion): cooling rate in the region from surface to
mm in depth in temperature range from 750 C to 500 C. Cooling rates at head
side portion and lower chin portion are average figures of
right and left sides of rail.
*4 TCR = 0.05 x T (cooling rate at head top portion, C/sec.) + 0.10 x S
(cooling rate at head side portion, C/sec.) + 0.50 x J (cooling
rate at lower chin portion, C/sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a
depth of 30 mm (head inner portion) from head top surface.
Table 33 .
Symbol Steel Value 2 CCR 4 CCR Rolling Time up Heat Heat treatment
conditions of head surface Microstructure *5
of CCR length to the treatment
*1 start of conditions
heat of head
treatment inner
of head portion
portion Cooling Cooling
rate Cooling rate Cooling rate Value of
rate *2 at head top at head side at lower chin
TCR *4
(value of portion *3 portion *3 portion *3
ICR) T S
A
(m) (sec) ( C/sec) ( C/sec) ( C/sec) ( C/sec)
Head top
Pearlite
0.30 0.70 portion
(Insu- (Insu- Pearlite +
185 80 0.39 0.78 1.56 198 198 2.0 1.0 1.0
fficient fficient Head inner pro-
cooling) cooling) portion eutectoid
cementite
,
Pearlite +
0
Head top
2.80 bainite +
---- portion
186 80 0.39 0.78 1.56 185 178 1.25 6.0 5.0 4.0
(Over- martensite 0
N)
cooling) Head inner
Pearlite
.....3
0.
portion
to
_
_______________________________________________________________________________
__________________
Head top
01
Pearlite
0
Compare- (Insu-
(Insu- Pearlite + N)
187 81 0.81 1.62 3.24 185 165 4.0 1.0
2.0 I 0
tive heat fficient
fficient Head inner pro-
treatment cooling)
cooling) portion eutectoid i--, I-,
1
methodcementite
NJ 0
Head top 1
Pearlite
0
188 81 0.81 1.62 3.24 175 150 1.75 5.0 5.0
6.0 (Over- Pearlite +
Head inner
cooling) bainite +
portion
martensite
1.05 2.10
(Insu- (Insu- Head top
189 82 1.24 2.48 4.96 160 135 4.0 4.0 3.0
Pearlite
fficient fficient portion
- -
cooling) cooling)
_
_
Pearlite +
5.00
---- Head inner pro-
190 82 1.24 2.48 A.96 160 120 2.35 10.0 10.0 7.0 (Over-
portion
eutectoid
cooling)
cementite
*1 CCR ( C/sec.) = 0.6 + 10 x ([cisC] - 0.9) - 5 x ([%C] - 0.9) x (%Si] -
0.17(96Mn] - 0.13(%Cr]
*2 Cooling rate ('C/sec.) at head inner portion: cooling rate at a depth of 30
mm from head top surface in temperature range from 750 C to
650 C.
*3 Cooling rates at head surface (head top portion, head side portion and
lower chin portion): cooling rate in the region from surface to
mm in depth in temperature range from 750 C to 500 C. Cooling rates at head
side portion and lower chin portion are average figures of
right and left sides of rail.
*4 TCR = 0.05 x T (cooling rate at head top portion, c/sec.) + 0,10 x s
(cooling rate at head side portion, C/sec.) + 0.50 x LT (cooling rate
at lower chin portion, C/sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a
depth of 30 mm (head inner portion) from head top surface.
Table 34
Symbol Steel Value 2 CCR 4 CCR Rolling Time up Heat Heat treatment
conditions of head surface Microstructure *5
of CCR length to the treatment
*1 start of conditions
heat of head
treatment inner
of head portion
portion Cooling
Cooling rate Cooling rate Cooling rate Value of
rate *2 at head top at head side at lower chin
TCR *4
(value of portion *3 portion *3 portion *3
ICR) T S A
(m) (sec) ( C/sec) (
C/sec) ( C/sec) ( C/sec)
=
_______________________________________________________________________________
____________________________________
Head top
250Pearlite
portion
(Time too
cementite
Head inner trace pro-
formed)
portion eutectoid
cementite
0
_______________________________________________________________________________
_____________________ --
Head top
Pearlite
0
0.95 2.00 portion
N.)
.....3
(Insu- (Insu-
192 83 1.13 2.26 4.52 145 80
6.0 2.0 3.0 Pearlite + 0.
fficient fficient to
Head inner pro-
01
cooling) cooling) 0
Compara-
portion eutectoid w
tive heat
N.) cementite
_
_ -
I
treatment
0
Head top
method
I-,
1.00 2.10 portion Pearlite
(....)
0
(Insu- (Insu-
1
fficient fficient
Head inner pro-
'-
cooling) cooling) I o
portion
eutectoid
cementite
_
245
Head top
Pearlite
(Excessive
portion
194 86 2.66 5.32 10.64 rail 15 2.25 12.0
8.0 14.0 8.40 Pearlite +
length,
Head inner trace pro-
rail ends
portion eutectoid
overcooled)
cementite
*1 CCR ( C/sec.) = 0.6 + 10 x ([%C) - 0.9) - 5 x ([%C] - 0.9) x P6Si.) -
0.17(%Mn] - 0.13(%Cr]
*2 Cooling rate ( C/sec.) at head inner portion: cooling rate at a depth of 30
mm from head top surface in temperature range from 750 C to 650 C.
*3 ,Cooling rates at head surface (head top portion, head side portion and
lower chin portion): cooling rate in the region from surface to 5 mm
in depth in temperature range from 750 C to 500 C. Cooling rates at head side
portion and lower chin portion are average figures of right and
left sides of rail.
*4 TCR = 0.05 x T (cooling rate at head top portion, C/sec.) + 0.10 x s
(cooling rate at head side portion, C/sec.) + 0.50 x J (cooling rate at
lower chin portion, C/sec.)
*5 Microstructures are observed at a depth of 2 mm (head top portion) and at a
depth of 30 mm (head inner portion) from head top surface.
CA 02749503 2011-08-10
- 131 -
Industrial Applicability
The present invention makes it possible to provide:
a pearlitic steel rail wherein the wear resistance
required of the head portion of a rail for a heavy load
railway is improved, rail breakage is inhibited by
controlling the number of fine pearlite block grains at
the railhead portion and thus improving ductility and, at
the same time, toughness of the web and base portions of
the rail is prevented from deteriorating by reducing the
amount of pro-eutectoid cementite structures forming at
the web and base portions; and a method for efficiently
producing a high-quality pearlitic steel rail by
optimizing the heating conditions of a bloom (slab) for
the rail and, by so doing, preventing the generation of
cracks and breaks during hot rolling, and suppressing
decarburization at the outer surface of the bloom (slab).
